Computer freezes all the time?

My computer locks up and loops a millisecond of whatever audio it was playing at many different times, when watching videos in youtube and WMP, while playing games, while randomly surfing the internet, when syncing my iphone, when running virus scans, etc...it wont freeze if no ones on it because i leave it on all day and its fine but as soon as i start using it again it takes about 5 mins for it to freeze. i have windows XP home SP3 and have done everything from cleaning my registry to buying fans to keep it cool...i dont know what to do anymore. also all the lights on my keyboard (caps lock etc) do not respond to hitting the keys until i reset.