[Cody Carey]

May I suggest replacing your current Virus software with ZoneAlarm Internet Security Suite?

It is a VERY nice all-around computer protection program, and I think that it only costs something around $15/Yr for updates. If you don't like paying anything, they have a free 15-day trial for download that you can install and run every once-in-a-while to clean up your computer, and then just go back to using your old virus coverage.(If you do this, make sure the programs aren't running at the same time)
Also, Defrag regularly and try to avoid messing with registry cleaners, as there is a possibility of them harming your computer more than it already is, although if you invest in a very useful program called TuneUP Utilities, It makes registry backups and gives you the ability to boot windows directly from the backups, which is useful indeed.
If your computer stays slow after you run a trusty Virus/malware program, and you know approximately when your problems started occurring, you can always do a search yourself:
Open the windows default search utility and search for "*.exe" under the advanced tab,set the "Created" date to just before you first started having problems and search. If your search returns a number of programs with names such as "aaaaaaaa.exe", or "555555555.exe" in your Windows or System folders,then you are probably still infected with some virus or malware, in which case you should look for a better virus removal program. You probably don't want to delete random files from your hard drive, however... so trying to take care of it yourself may not be a great Idea.

Sorry if this is a little late, But ZoneAlarm Internet Security Suite Has never let me down...