Simply boot from the WinXP CD and be careful not to be tricked into entering “rescue mode” because that is NOT where you want to be (at a prompt, all dressed up with nothing to help you fix your problem). Windows will tell you that it is looking for existing OS / Windows installations and you hope that it will find yours. If it does, it will give you the option to “repair” that installation of Windows.

When you are down on your luck, this is a godsend. All your settings, your documents, your programs, and your registry will be maintained. Windows will reload the core system files to get you back on your feet. This will take you back to the original files from your install CD, so you will have to apply every update / service pack released since then.

So if you are lucky, you can salvage the deal before it goes raw. If the bitwise gods do not shine so well on you, there is not much else you can do besides reload windows and loose all your settings / program registry entries.