Frozen computer

My computer keeps freezing up, my mouse keeps flickering or dissapearing completely and I am constantly having to use the reboot button. This only seems to make things worse. I wonder if I have too many programs installed. Things didn't start happening untill I started downloading programs of which some I can't even run because portions were downloaded into the wrong files. I ran maintenance wizard and checked for viruses and they came out clean, but the "freezing" problem was even worse! I downloaded "Bonzi-Buddy" and now he only moves frame by frame. I'm suprised I'm even in the internet right now! Please help me - I'm ready to send this "bain of my existence" back!

P.S.- I have a Dimension XPS T500
P.P.S.- Is it true that if Dell is offering what you bought for less, that they'll reimburse you within 30 days of your purchase or is that just a bunch of CRAP?