Shoutbox

Download this program, execute it and tells us what you see (CTRL+C). Maybe there's something at startup that shouldn't be there. Spyware can slow down your PC too, so be careful with that...

Buying more memory will help, but it's not necessary (so don't worry if you can't spend money on it, you said it was working fast before).

You can try reinstalling Windows over the current installation and then, if it doesn't solve the problem try making a fresh install (when nothing works this is the best solution, even if you have to reconfigure everything ).

I don't like programs like Tuneup, they tend to make more problems than solutions. I've never tried Tuneup, so I'm going to download it now and install it on a test machine.