seems to be some sortofa bootskin problem posibly.
try to boot in 'last known good configuration'.
try to do a system restore.
if they dont help, go to safe mode.
did u install any new applications/ programs recently ? if yes uninstall them.
then check up ur start-up entries. if u can, disable the non-microsoft ones' for the moment. u can use the xp tool, or else install winpatrol (in safe mode itself) and use it.
in safe mode also use 'ccleaner' or other such cleaner to clear ur pc junk.
hopefully it shud help. if it doznt, do revert pls.