Computer slow due to possible infections

My computer is getting more and slower daily. I have an old computer and about last year I just upgraded upgraded RAM to 1.25 GB, and did a clean install of windows XP. I don't like to do all those once again. There has to be something causing this slowdown. I was referred to do a scan with anti spyware by my teacher. I won't take any definitive action until after I hear from someone. Any help would be GREATLY appreciated. Thank you!