Windows XP keeps on disc checking. Won't boot.

I left my computer on overnight and switched it off at the computer power switch the next morning. When I started it up again later that day it came up with the "run in normal mode or safe modes" page. I clicked on Normal, the Windows startup page appeared as normal, but for only about 20 seconds and then it went to the C/: disc-checking page. I cancelled that and it rebooted only to come up with the NORMAL/safe mode page again. I chose "Last good session" and it came up with the disc-checking page again. I let it check the disc this time but it still repeated the cycle. I've tried all the otrher options on the Normal/Safe page but it still keeps disc-checking. How do I get out of this vicious cycle?