You definately must have an always-on antivirus tool running to ensure the health of your PC
(using a real-time AV tool only one at a time). There are some good free Antivirus programs that are decent, including AVG and Avast!.
AVG: http://free.grisoft.com/doc/1
Avast: http://www.avast.com/eng/avast_4_home.html
Please run the
READ FIRST Procedures found here, if you have not already done so and post a HijackThis (HJT) LOG:
Read This First - IMPORTANT Instructions
DO NOT FIX ANYTHING in the HJT LOG unless advised to - most items that appear in the log are harmless or even needed.