When it does that, have you tried pressing Ctrl-Alt-Del to see if that will shake it awake? It's a bit less drastic than powering it down, and often solves minor glitches.
I don't remember if XP Home has the "log" feature. Try Start - Control Panel - Administrative Tools - Event Viewer - Application and see what red flags are there. That might help.
Other than that, I have no ideas. I've never had one of mine do that, but I rarely switch users anyway.