Use a good antispyware. Ad-aware, Ewido are free spyware removers. You can download free softwares at http://fixit.in/antivirus.html and http://fixit.in/spywareremover.html

Install update and run Superantispyware
http://www.superantispyware.com/
Easily remove over 100,000 pests such as AntiVirGear, VirusProtectPro, DriveCleaner, SmitFraud, Vundo, WinFixer, SpyAxe, SpyFalcon, WinAntiVirus, AntiVermins, AntiSpyGolden and thousands more!
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"But I'm still getting that small icon at the bottom of my screen about spyware."

Next time we need to know exactly what the pop up says, the web page it refers you to, and the product it tries to sell you. This will allow us to determine the correct removal tool needed.

If you follow all the following steps it should get rid of your problem and prevent future problems. All programs listed are free.
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Update your Full Time (Active) antivirus and run a full scan.

If you do not have active virus protection, install only one. All are excellent.

AVG Antivirus 7.5 Free Edition
http://free.grisoft.com/freeweb.php/doc/...
http://www.download.com/AVG-Anti-Virus-F...
or
Free antivirus - avast! 4 Home Edition
http://www.avast.com/eng/avast_4_home.ht...
or
Avira AntiVir PersonalEdition Classic
http://www.free-av.com/
http://www.download.com/3120-20_4-0.html...
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Update your Full Time (Active) spyware protection and run a full scan. If you do not have active spyware protection, install:

Windows Defender
http://www.microsoft.com/athome/security...
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Install the following four programs and run weekly or at least monthly. You need all four. They are not a substitute for full time spyware and virus protection. They will greatly increase your protection. Install and run now.

Ad-Aware SE Personal (update + full scan)
http://www.lavasoftusa.com/products/ad-a...

Spybot Search & Destroy (update + immunize + scan)
Do not enable Tea Timer and SDHelper
After installation: update + scan + immunize
http://www.safer-networking.org/en/mirro...

SpywareBlaster: Update then open and click “enable all protection”.
http://www.javacoolsoftware.com/spywareb...

SUPERAntiSpyware free version: (update + scan)
http://www.superantispyware.com/
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Install:

CCleaner:
http://www.ccleaner.com/
Do not install optional toolbar.
Removes tracking cookies, unneeded files, history
In options.
Set to run when computer starts.
Place cookies you want to keep in save list
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Install either one not both:

McAfee Site Advisor: Internet Explorer and Firefox
http://www.siteadvisor.com/
McAfee SiteAdvisor helps protect you from all kinds of Web-based security threats including spyware, adware, spam, viruses, browser exploits, and online scams.

TrendProtect™
http://www.trendsecure.com/portal/en-US/...
TrendProtect is a FREE browser plug-in that helps you avoid Web pages with unwanted content and hidden threats.
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Note if a scan detects a problem but is unable to remove, start the computer in safe mode with the internet line disconnected and run a full scan.

In severe cases your system restore files will also be infected. In these cases you will need to turn off system restore to prevent malware hiding in the system restore files and reinfecting the computer during removal or during a future system restore. Turning off system restore deletes the system restore files.

Right click on "my computer"> Properties > System Restore Tab > Check box turn of system restore

After the malware is removed turn on system restore.
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Run this time:

CWShredder: run
http://www.trendmicro.com/cwshredder/
Shoot The Messenger
http://www.grc.com/stm/shootthemessenger...

VX2 tool for Ad-Aware and run tool (Install and run)
http://www.lavasoftusa.com/support/secur...
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Special Removal Tools
Run only if indicated

Roguefix.bat
http://www.internetinspiration.co.uk/rog...
SmitFraudFix
http://www.geekstogo.com/forum/How_to_us...
Vundo Fix and
VirtumundoBegone (if VundoFix does not work)
http://www.bleepingcomputer.com/forums/t...
Vundo Rootkit Removal
http://wiki.castlecops.com/Vundo_Rootkit...
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Run this time and monthly:

Microsoft Update:
Run in "Custom Mode". Install everything, reboot and repeat until nothing is left to install.
http://www.update.microsoft.com/microsof...

Microsoft OneCare Live, run “full service scan”
Updates windows, virus and spyware scan, disk cleanup, disk fragmentation (if needed), backs up registry and then cleans registry, and checks for open firewall ports
http://onecare.live.com/site/en-us/defau...

Malicious Software Removal Tool (run “full scan”)
http://www.microsoft.com/security/malwar...
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Rootkit Removers
Pick any 2 install and run one each month.

AVG Anti-Rootkit
http://www.grisoft.com/doc/products-avg-...
F-Secure BlackLight
http://www.f-secure.com/blacklight/
Panda Anti-Rootkit
http://www.pandasoftware.com/products/an...
http://www.pandasecurity.com/homeusers/d...
http://www.download.com/Panda-Anti-Rootk...

Sophos Anti-Rootkit
http://www.sophos.com/products/free-tool...
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Online Free Scanners:
Run Trend Micro, Kaspersky, and Panda Scan now.
Run a different one each month.


BitDefender Online Scanner
http://www.bitdefender.com/scan8/ie.html
CA eTrust® PestScan
http://pestpatrol.com/pestscan/index.htm
eTrust Antivirus Web Scanner
http://www3.ca.com/securityadvisor/virus...
F-Secure Online Virus Scanner
http://support.f-secure.com/enu/home/ols...
Kaspersky - Free Online Virus Scan
http://www.kaspersky.com/virusscanner
McAfee - Free Scan
http://us.mcafee.com/root/mfs
Panda Software - ActiveScan
http://www.pandasoftware.com/activescan/...
Symantec (norton) - Security Check
http://security.symantec.com/sscv6/defau...
Trend Micro™ HouseCall
http://www.trendmicro.com/hc_intro/defau...

The presence of Anti-virgear phony alerts and warnings usually indicates a Smitfraud infection.

The standard tool to remove this pest is SmitfraudFix. The full removal procedure and the free removal tool is here:
http://siri.urz.free.fr/Fix/SmitfraudFix...

Almost all "general purpose","all-in one" spyware and virus removal programs such as AVG, Norton, or Spybot S & D cannot remove smitfraud completely.
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You may have caught this infection by attempting to watch a video at some website. The site said you "needed" a codec so then you downloaded it to watch the video. This was actually a ruse. A scam if you will. It was not a codec at all. (The video would have played fine without that "Codec")! What you could have actually downloaded was a Trojan called "Zlob".

This trojan then "phoned home" and installed the popup saying "system alert" adware on your computer with out your knowledge.

The fake alerts do not usually appear until after you reboot your computer.

It is almost impossable that your download of the Gimp had anything to do with your Smitfraud infection. Gimp is open source, so if the code were infected, it would have been discovered long long ago.

Good luck.