If you can figure out how to get to the Windows XP recovery console, you can use the command "fixmbr" to restore a previous version of the MBR on that partition. I am NOT sure just what this will do since you have an altered MBR to begin with. I get to the recovery console by booting the XP install CD and booting into the recovery console. There are other ways, since you said your system came without recovery or Install discs. The Recovery Console might even be installed on your machine. You can download the Windows XP XP Boot Disks can be found at the following links. Be sure to use the right Version, Pro or Home and SP-2 if it is up to date:
http://www.microsoft.com/downloads/d...displaylang=en
http://www.microsoft.com/downloads/d...displaylang=en
Here are a few links to help you out.
http://www.pcworld.com/howto/article...,112479,00.asp
http://support.microsoft.com/default...b;en-us;307654
http://www.wown.com/j_helmig/wxprcons.htm

I am sure there are many more ways to fix your system, but you should do some research before doing ANYTHING and making your situation worse. If you need to get out of the hole you have dug for yourself, the first thing you need to do is to STOP digging! There is a wealth of information out there and most of it can be found with Google or just searching within the Micro$oft site.

One recommendation I will make about this situation. You can restore the system back to it's original configuration of running Windows XP. Recover whatever files you need to keep BEFORE restoring your drive. Then save the experimentation with various versions of Linux, multiboot installations, etc, for another system that you use JUST for that type of experimentation. When something goes bad, as it has in this case, you can then wipe it clean and start over without breaking a sweat. A second system with enough horsepower to run Knoppix or most other versions of Linux can be had for very little money AND you can use a KVM to switch your monitor, keyboard, mouse, and even sound between the systems. Just remember to back up everything you might need BEFORE doing ANYTHING!