How do you know or find it

[DLeonR]

How do you know if you have a virus in your register files and how do you know where to look to find it. I was reading in my A+ Guide to Managing and Maintaining Your PC and it talked about that but it did not say how to find it. The registry in windows XP has six branches, I just thought it would be a task trying to find something that you don't even know the name of or anything like that. My friends are always bringing there computer to me for all kinds of problems, to include viruses. I usually help them because I have anti-virus programs, but sometimes that don't work very well.
Any ideas on how you go about this?