Scenario 3: An error message appears, before you can see the windows logo

If you have an error before windows loads, so before the windows logo appears, the error message most likely comes from the BIOS. Refer to "scenario 2" to identify your BIOS and check here what the error means.

It can also come from the boot loader, the most known error on XP is "NTLDR is missing". Check this article if you want to know what to do in that case: http://www.computerhope.com/issues/ch000465.htm

If you have an error in Windows Vista like “bootmgr is missing” or “winload.exe is missing”: try the windows vista start-up repair. http://windowshelp.microsoft.com/windows/en-us/help/5c59f8c1-b0d1-4f1a-a...

An error this early could also come from a corrupted registry. Although it is probable easier to Reinstall Windows, you might have a reason worth the effort to try and recover your windows XP form a corrupted registry. Check this article for more information:
http://www.wikihow.com/Recover-from-a-Corrupted-Registry-That-Prevents-W...