After the Dell Splash screen hit F8. This will be the Windows Startup menu. Choose disable automatic restart on failure. Then let it boot. Then you will be able to see the blue screen error. Note it. If it says "Incaccesible Boot Device", then run the HD diags like above post indicates. If the drive passes the diagnostics, then you need to run check disk to try and repair. To do that you'll need the Windows XP CD you got from Dell with the PC.
Here is link on how to get to the recovery console:
http://www.wikihow.com/Use-Windows-Recovery-Console-from-XP-CD
Follow to step 5.
Once you can type a command in, type this: "chkdsk /r" without the quotes. It will run for a while and then you will get the command prompt again. Type "exit" and your machine will reboot. Hopefully it will boot to desktop.
If message is anything other than what I said, repost the error.