this is my guess,
somewhere in the registry of your system its gone wrong,
as your computer boots up hit F8 until you get a list of options.
choose "safe mode with networking"
(select administrator if it is displayed)
let it boot up,
go to start>run> type "msconfig"
go to the "boot.ini" tab and click "check all boot paths".
note: if you have a windows XP disc, then i would advise you to reboot with that inside, and do a repair.
your files WILL NOT be lost.