Calling all computer experts!

I've been having computer issues for the last few months. Certain days are just terrible and others aren't so bad. Sometimes I run soooooooooooo slow!! I've cleaned so much stuff out and I'm still having these issues. I feel like dumping my entire system and reinstalling Windows ?? I think I have XP now but I thought about buying Vista.
If I dump and load the new Windows will my problems go away? Sorry if this is a dumb question but I seriously have no idea. I wasn't sure if the problems would be hidden in there no matter what I do.

Thanks for any help.