i had this a few weeks ago
i fixed it myself
if u do a repair u can pull all the data onto a external meory but u will lose all ur program links and will need to reinstall

however u can fix this 2 ways
norton have a disc u can by that has a recovery console so u can do it without losing data

2 a little bit of comp fiddling - insert xp disk and u should get the option to log on (cant remem if u go to repair first) it will look like cmd screen (black with white writing)
type in help
there are a few scans to do which can find the corrupt or missing file ect.
u need to replace it with the one on hte disk
then log off and reboot with out the disk
should work fine

that may be a little too complex and a vague explanation but it can be done with out causing any damage no lost files or program links