Several problams may be causing this, but primarily it's a missing or corrupt Master Boot Record (MBR).

First, power up your PC and insert your Windows CD-ROM. Boot from the CD. The startup screen will offer you an option to Repair Windows. Yes, you want to do this. Follow the instructions on screen untill you get to a Comand Prompt. Then type "fixmbr" (without the quotes). Windows will then install a new MBR. Restart your PC and it should load to your desktop.

If this doesn't work, remove the hard drive from the PC, connect it to a second PC as Slave, backup all you personal data (documents, pics, etc.) and reinstall the hard drive back into the origional PC. Try once again to restart the PC.

If all else fails, you need to reinstall Windows new.

Yes, you will loose any information that you already have on your drive. That is why backing up you data regularly is so important.