Photoshop problem

I keep getting this werid problem that I have never goten before. It keeps saying somthing about a scratch volume thing, and it also says photoshop wont perform right. It used to work fine now, it lags real bad. Ive unistalled it reinstalled and it still lags. It never used to. I got 512 ram if that helps. Any ideas whats up?
thanks

First, What exactly is it saying about the scratch disk?

Is it full?
Problem writing to it?
What?

Second, is your scatch disk you primary Hard disk?

If your scratch disks are full, try deleting your photoshop temp files. Sometimes they can build up in your hard drive taking up too much space. I'm thinking they should be in:

C:\documents and settings\(your user name)\local settings\temp\

But local settings is a hidden folder. (but this only applies if you are running a PC)