Firefox is good...have you cleaned your PC since you found the viruses that you picked up before?
Virus/Spyware Removal Instructions:
Download, install, and update antivirus and antispyware programs (found below). Turn off System Restore off (Right click My Computer, to Properties, go the Restore tab, check the box to Disable system restore). Reboot into Safe Mode (pressing F8 on power up). Run the installed antivirus/spyware. After you have verified they are removed turn System Restore on (uncheck the box) and make sure all of your Windows Updates are completed.
Reg Cleaner
http://www.pcworld.com/downloads/file/fid,64953-order,1-page,1/description.html
Ad-Aware
http://www.download.com/Ad-Aware-2007/3000-8022_4-10045910.html?tag=lst-1
Spybot
http://www.download.com/Spybot-Search-Destroy/3000-8022_4-10122137.html?tag=lst-1
Spyware Terminator
http://www.spywareterminator.com/download/download.aspx
SuperAntiSpyware
http://www.superantispyware.com/superantispywarefreevspro.html
Free Online AV Scan
http://www.pandasecurity.com/homeusers/solutions/activescan/
Clam AV
http://www.clamav.org/download/
AVG
http://www.download.com/AVG-Anti-Virus-Free-Edition/3000-2239_4-10320142.html?tag=lst-1
Threat Fire (Behavior analysis, so can be run side by side with antivirus without causing problems)
http://www.pcworld.com/downloads/file/fid,64144-page,1-c,antivirus/description.html
PC Cleanup and Maintenance
There are a few things that should be routinely done to your PC to maintain it, I typically recommend at least once a month depending on your usage. These things include (but are not limited to):
Perform Microsoft updates (they are released on the second Tuesday of every month). Be sure to click on Custom not Express.
http://www.update.microsoft.com
Disk Cleanup
http://www.stevengould.org/index.php?option=com_content&task=view&id=15&Itemid=69
http://www.CCleaner.com/
Defragging your hard drive
Either with the Windows Defrag tool or Defraggler
http://www.defraggler.com