Depends on the kind of virus/trojan. See "Computer Viruses:
Description, Prevention, and Recovery"
(http://support.microsoft.com/?kbid=129972) and "List of Antivirus
Software Vendors" (http://support.microsoft.com/?kbid=49500). Also
download and use Ad-aware (http://www.lavasoft.com) for to find
malware.

Hi

Do you know the name of the virus that has infected your system?  Try an
online scan for a second opinion:

http://www.pandasoftware.com/products/activescan/com/activescan_principal.htm

Hi,

Free virus removal tools:

http://vil.nai.com/vil/stinger/
http://www.emsisoft.com/en/
http://free.grisoft.com/doc/8/lng/us/tpl/v5/nid/3001#3001
http://www.f-secure.com/download-purchase/tools.shtml

Also, you may use this free on-line scanner:
http://housecall.trendmicro.com/

Symantec also distributes many free removal tools that are virus-specific if
you know the name:
http://securityresponse.symantec.com/avcenter/tools.list.html

I would strongly suggest that you work in Safe mode to facilitate an easier
removal. How to start in Safe mode:
http://www.rickrogers.org/fixes.htm#Safe%20mode