Hi,
I suppose you tried the procedure as described in this article
How to recover from a corrupted registry that prevents Windows XP from starting
Unfortunately,it doesn't tell you what to do if this doesn't work.

I've seen a lot of people with this problem having to re-install from scratch.
I would do this as a last resort effort,but failing a better solution,you may have to.
You could try one of the various rescue disks.
Since it seems you have access to another computer,if you decide to reinstall
and want to save your data, install the unbootable drive as a slave and
save what you want on the main drive.
Wipe the slave drive and reinstall on your pc.Then do the reinstall.
The data can be transferred later via USB.

Let's see if you get some other replies.There are a lot of clever people on this forum