Remember that there’s a difference between a virus, spyware and adware.
*Virusses attack system files and try to harm your PC, whereas
*Spyware installs unsolicited software on a user’s PC not necessarily intended to harm your files, but usually to collect/steal information from you and
*Adware is unsolicited software used to advertise products or generate income usually with browser pop-ups.
Most anti-virus programs don't remove spyware/adware and even if it does I always find a specialist product works best. So it is always recommended that a PC be running a suitable and up to date antivirus program and one or more adware/spyware checkers at all times.
As indicated in my website http://pgg.co.za/ I find the following free solutions to provide most effective protection for;
*Virusses - PC Tools Antivirus Free Edition - http://www.pctools.com/free-antivirus/
*Spyware - Spyware Doctor Starter Edition - http://www.pctools.com/spyware-doctor/google_pack/
*Adware - Ad-Aware 2008 Free - http://www.lavasoft.com/products/ad_aware_free.php