|Your problem can be caused by a hard drive that is in the process of failing.|
Check your hard drive with the manufacturer's diagnostics.
See the latter part of response 1 in this:
If you don't have a floppy drive, you can get a CD image diagnostic utility from most hard drive manufacturer's web sites, but obviously you would need to make a burned CD, preferably a CD-R for best compatibility, on another computer if you need to.
Seagate's Seatools will test (almost) any brand of hard drive.
Do the long test.
The bootable Dos versions of SeaTools can be used even if Windows is not working properly.
If the hard drive itself tests okay, any data problems found can be fixed, one way or another.
If the hard drive itself DOES NOT pass the diagnostics, you need to replace it, and replace at least the Windows installation if not all of the original brand name software installation, and copy the personal data you don't want to lose to elsewhere if that's still possible.
If the hard drive itself passes the diagnostics....
The best way to fix your problem is to run a Repair installation of Windows procedure, often called (incorrectly in my opinion) a Repair Install, which will not harm the personal data you have added already on the partition Windows was installed on.
However, you need a suitable Windows XP CD to do that procedure.
Some Dell computers that originally had XP on them came with the equivalent a Windows CD when they were new. It's got Dell labeling and "XP Home SP2 Re-installation CD" or similar printed on it .
Do you have that CD ?