DJ if you have no luck with that here is an excellent Guide:
Repair Install of XPI suggest reading thru it to become familiar and also print out a copy for reference...
Done properly all your data and apps will still be there and XP will be fixed...
Done improperly all your data and apps will be gone but you will have a fresh clean install of XP.
Best of Luck to you and keep us posted...
patio.
P.S. This is provided we aren't looking at a hardware issue here.