What real-time virus detection program are you running currently?
If you don't use any virus deterrent software, I'd strongly recommend installing one, whether or not you have a virus at the moment. Here are a few good options...
avast!
AVG
Avira AntiVir
A-squared Free
Your problem might not be a virus. If it is more along the lines of adware, spyware, malware, hijacker, then one of these detection/removal programs will prove extremely beneficial...
Ad-Aware (Anniversary Edition)
MalwareBytes' Anti-Malware
Spybot - Search & Destroy
SpywareBlaster
Spyware Doctor
Spyware Terminator
Trend Micro Hijack This
For something that does antivirus
and antimalware
and antispam real-time detection and removal...
ESET Smart Security *only a limited trial version is free
For something that cleans your computer of unnecessary files, junk and clutter (malware, temporary files, cookies, incorrect or unnecessary registry entries, etc.), then these could be helpful [be careful with 'em though, as you don't want to remove anything important accidentally]...
Ace Utilities *only a limited trial version is free
Advanced SystemCare Free
ccleaner
Wise Disk Cleaner