Win XP? What PC model?
You can try this:
Reboot and press F8 before Windows starts to load. Choose Last Known Good from the menu. If that works, shut down normally and reboot normally.
If that doesn't help, you may want to run chkdsk on the hard drive. Reboot from the Dell Windows Reinstallation CD. Press R at first screen to open the Recovery Console. At C:\ prompt, type in:
chkdsk c: /r
Press Enter
Go take a nice long nap!
When it's done, remove the Windows disk and reboot. If you're lucky....
If that doesn't help, you may need to run a Repair/Reinstall of Windows. Personal files will be ok, but you will have to reinstall a lot of Microsoft updates and hotfixes again. In order to run the repair, the Windows disk must have the same version of the OS (eg, XP Home or XP Pro) and the same Service Pack as installed on the hard drive (eg, SP3).
And be sure to back up your personal files on external media, in case this was the first indication of a hard drive failure.