Tim, it's always easier to give some more info on your puter, like:
Mac or Win?
Which OS? (OS9, OSX.., Jaguar, Win98SE, WinXPHome,...)
Did you install the latest security patches or updates of your OS?
Processor speed?
How much RAM?
Free space on the HDD?
If you have a virus, most probably your virus scanner wasn't updated lately.
If I wouuld have a serious virus infection, I would uninstall everything and start anew. Make backups if still possible, but take very good care that the virus doesn't hide somewhere unexpected on one of your backup CD's.
Imo, a virus scanner works preventive, but should not be used to get a really dangerous virus out of your system.
Main sources are e-mail, and shared files with other puters. There are some malicious codes for sites too, but these are exceptional.
Another thing is spyware like Gator etc. Never trust any gator!!!!! You can find them and delete them from your system with AdAware:
www.lavasoftusa.com . Get it from Tucows or so and download the latest ref file. Put this reffile after unzipping in the ProgramFiles>Lavasoft>AAW6 folder, and say yes to overwrite. Then run and don't be surprised to get a whole list of parasites eating up your system's resources. Wipe them out. All of them.
Then run a defrag.
And finally, when possible, give PS a scratchfile on another HDD or partition than the one you installed your Windows on. That way it can't interfere with Win's own scratch file.