LinuxQuestions.org - HELP! partition table screwed up

Yesterday I wanted to empty my usb flash disk:
the command I wanted to use was the following:
dd if=/dev/zero of=/dev/sdb
instead, I mistyped and did this:
dd if=/dev/zero of=/dev/sda (I got used to typing sda)

I found out the operation took a bit long, and I didn't saw the light of my usb stick flicker, so I cancelled it.
It prompted that 1,5gig was written, then I found out it was my HD!
the hd contained:

[ sda1 - fat32 - win32 - 20gb ] [ sda2 - reiser4 - linux 2.6 - 20gb ] [ sda3 - swap - 1gb ] [ sda4 - fat32 - data - about 100gb ]

So, my partition table is screwed up, and my first fat32 filesystem is also.
The fact that win32 is gone is not a problem at all, I didn't do anything with it at all, but the fact that the partition table is gone, IS a big problem, because now, I cannot access any of my filesystems anymore.

my question is:
Is there a tool that can find out where on my hd a filesystem starts and ends ? so I can rebuild a partition table, so I can access the 100gb data disk (the data on it is essential to me), and maybe sda2 ? (it took me quite long to get such a nice configuration)

The data on sda4 really is essential to me, it would be a real pain to see me formatting the whole disk again, while it is recoverable...

please give me a clue on how to do this, any help is appreciated.. I need that data back...

thank you.
ps. No, I don't have a backup of my partition table, otherwise I would have already returned it.