cleaning malware & viruses off a computer with no AV package
have a Antivirus+antispiware[+firewall] cd ready.
first, I suggest, you have a copy of at least Clam Antivirus handy (Windows, Mac, Linux). Please note that this particular AV package does not have shields/a real-time virus scanner. But it does clean the machine. Also with this package, you must configure it to schedule scans on your hard disk(s), and also update fully automatically at logon, and move infected files to quarantine folder.
OR, run MRT.exe
peridocally, Microsoft Automatic Updates sends the Malicious Software Removal Tool down the wire. it is about 26MB in size, and you must run it manually to scan your computer to remove malware/viruses/worms, if any. It might not be a complete tool, but it cleans out the big ones.
scan your machine with the Microsoft Windows Malicious Software Removal Tool. [windows-logo-flag-key]-R c:\windows\system32\mrt.exe[Enter] do full scan and wait 8+ hours for report.
Commercial (pay-for) AV packages
Microsoft Security Essentials, available from microsoft.com $free
No realtime scanner or shields. only thing it can do if you configure it so, is scan and clean folders such as c:\ periodically. high detection rates, good cleaning. mcafee and probably norton will remove clamwin.