Note that with any serious system infection, you can not guarantee that the malware can be fully removed. That being said, you might be better off just backing up your important data and performing a complete system reinstall.
Anyway, if you haven't done so already, install an up to date anti virus software like Microsoft Security Essentials, AVG, Avast, etc. Do a complete system scan and see what you can remove.
You could also try to manually remove the malware. Below is something from a Symantec forum. But be aware of the possibility that this will probably not work perfectly on a running infected system.
Stop These System Tool 2011 Processes:
System Tool 2011.exe
Find and Delete These System Tool 2011 Files:
%UserProfile%\Desktop\System Tool 2011.lnk
%UserProfile%\Start Menu\Programs\System Tool 2011.lnk
Remove These System Tool 2011 Registry Values:
HKEY_CURRENT_USER\Software\System Tool 2011