Not sure if you solved this issue or not yet, I'm a new joiner to this forum and just noticed your thread.
I shrunk my Windows XP partition using Gparted (LiveCD), then messed around installing various Linux distros on subsequent partitions.
Everything was going fine until I attempted to install FreeBSD on top of my Ubuntu (basically I wanted to replace Ubuntu with FreeBSD for learning purposes).
I'm sure it was my error, but somehow I messed up the boot loader that I had previously installed OK.
Then, through another error (watch out for those componunding errors :-), I used a Windows XP CD (recovery console) and the Fixmbr command (probably a mistake).
The Fixmbr command appeared to be the cause of my "invalid partition table".
Fighting a panic urge, I searched around and finally located something called "Testdisk", which claimed to be able to scan a disk and rebuild a partition table.
This comes with various rescue CD downloads; the one I chose was at http://rescuecd.pld-linux.org/
Using the instructions provided (and lots of Googling), I was able to restore the partition table so that I could at least boot from the original Windows. Nothing was disturbed on that install, which was nice. I had a backup, but wanted to see if I could recover without.
Sorry if this post is long-ish, but wanted to give you the background as well as the info. Hope it helps.