I don't think you can, and XP's search is next to worthless anyway, so it wouldn't make a lot of difference either way.
My advise would be to seek out other file searching programs. Google Desktop would be one, though as I understand it, it's not much of an improvement. Do a little searching on your own to find others and compare your options.
Can someone tell me hpw to change my Windows XP search to the old WIN98 search?