Computer not starting (constant restarting)

Hello. I am having a hard time today...not sure if I
should post this here or elsewhere. Well...here goes...
I cannot start my computer. Everytime I start it, it goes
to the Windows XP logo and then my computer restarts
(turns off, then back on). This can continue for
hours... I can't get to my desktop. I have ALOT of files
on my computer that I have not backed-up yet. How do I
fix this without having the chance of losing anything?
HELP! please reply to . Thanks!
-David