Windows help, please…

Windows help, please…
: Lately, my Sony laptop (using XP Pro) has started freezing for no apparent reason. It comes back after about 30 seconds and a beep; I’m not sure whether hitting the old control-alt-delete works, but I keep hitting it. I’ve caught it at these moments running at 100 percent. But I have not caught which application seems to be doing it. I’ve gone into config and killed everything at start-up that I can. Anybody else experiencing this? Any suggestions short of rebuilding?