running slow.. how to scan for virus

Your problem has nothing to do with "viruses," and scanning for "viruses" would be a waste of time. Please start a new thread with a more descriptive title, and provide all relevant information, including your OS X version, any error messages you've seen, and what you've done, if anything, to try to resolve it.

As Linc points out, performance problems are not going to be caused by malware. Malware for Macs is extremely rare and should not be the first thing to think of in a case like this... or even the fifty-first thing. 🙂 See my Mac Malware Guide.

As for solving your performance problems, it's hard to say how that may be done without more information. There are many possible causes. Try running through the troubleshooting steps in my Mac Performance Guide.