Crashing?

Ever since I've tried to download the trial for Photo Shop CS6 Friday or Saturday I've been having problems with my computer. I don't know what it is as I've never seen it or heard about this problem before. I'm in Firefox and it freezes and after I close it NOTHING works, the computer won't even shut down. When I try to open a program I get a message saying it's not a Windows image and/or it's not properly installed. After I force the computer to shut down (bad, I know but it won't otherwise) everything is back to normal and working properly.

I'm running Windows XP.