Windows freezes

[Nicola]

My windows xp has started to 'freeze' every now and
again. I haven't installed any new software or hardware
recently, and I can't think of anything that I did at the
time it began. I don't get any error messages. Ctrl Alt
Del don't work, and neither does the 'off' button on my
laptop. I have to switch it off at the mains or remove
the battery to restart it.

Any ideas??????

Nikki