You will want first try to boot your computer into Safe Mode or using the Last Known Good Configuration to see if you can replicate a more descriptive error. Most likely you will probably get an error referencing the boot.ini, ntoskrnl.exe, or ntldr files.
Power on your computer and press [F8] after you get the Compaq splash screen - just keep hitting the button, if the computer beeps at you, then you know its registering the input. That should bring up the Alternate boot menu selection screen.
If you are unable to boot into Safe Mode or boot using the Last Known Good Configuration, refer to the link below: http://www.microsoft.com/resources/documen...r.mspx?mfr=true
The link above will explain how to use the fixmbr command line utility, it also contains related links to the Recovery Console information page as well as the fixboot command line utility information page.