You don't seem to know much about viruses
Ok here's what you should do.
Set up a firewall, go into your internet connection - properties > advanced
Get the following programs, use them maybe once a week:
Spybot search and destroy
adaware
AVG antivirus(gotta love the free)
Also exploit any antivirus your comp came with.
And as well as that do the following:
Clear your temporary internet files(cache for firefox), and history and such
Defrag your hard drive
system scan
The last few aren't antivirus, just helpful hints on speeding the comp up. However I'm not giving up my best way, which sped my comp up by at least 10x
Note: all of that can miss stuff, so don't trust it 100%. I've had it miss some malware, and you need to know your registry to clean yourself good. My advice, if you don't know it, don't **** with it