Both of these are free and will work with vista.
Download Grisoft AVG 7.5 Anti-Virus,Anti-Malware.(free)
AVG is a collection of anti-virus protection tools that gives you full protection against viruses, worms, Trojans and malware. AVG provides you with all that you need to be completely protected: including a tool for scanning your hard drive and e-mail, as well as a real-time shield to prevent infections.
Top Features
» Provides automatic update functionality
» Its a reliable anti-virus scanning engine
» Provides free Virus Updates for life
» Options to define file name extensions
» Real time protection with resident shield, email scanning
http://free.grisoft.com/doc/5390/lng/us/...
PC Tools AntiVirus Free Edition feature highlights
Protects your PC as you are working, surfing and playing
Detects, quarantines, disinfects and destroys Viruses, Trojans and Worms
OnGuard™ protects your computer against threats in real-time
Automatically checks for frequent updates against the latest threats.
http://www.pctools.com/free-antivirus/
*Never install more than one anti-virus on your PC, as they may conflict with each other. On the other hand, its always good to install more than one anti-spyware.
* You will probably recieve several answers to your question. Whoever helped you the most,please come back and give that person Best Answer.