Some software companies sell dell windows cds for around $20. If you have a valid coa sticker number, you may be able to use them with your pc and activate windows. Otherwise, get a new windows copy with valid coa. Compaq may also allow you to download their windows version if you have the serial number and pay them a nominal fee.
To clarify: This isn't a hardware problem. There is a file missing from your hard drive, and that is what is preventing Windows from starting.
If you have your Windows XP DVD, you might be able to use it to repair the system--start the computer with the Windows disc in the DVD drive.
If you don't have a copy of Windows on DVD, you might want to look into getting one, or borrowing one.
You can borrow one. What you buy when you purchase windows is the license to use it. Thats the COA sticker on the side of your case. Heck you could even download a copy of windows off the net and use your number. Repair install would be the quickest route. This usually happens when overclocking to far. Have you tried setting everything back to default?