So I bit the bullet and got a copy of snow leopard delivered this morning.
Apparently it can’t install because the computer can’t start from my hard drive… Well obviously that’s false.
The problem seems to be related to partitioning, I did have a triple boot, leopard, xp and ubuntu. So after reading a few articles about this particular problem I started playing around (tweaking the disk size won’t work because of some bizarre journalling problem apparently, or mediakit something something depends on when you run it), deleted ubuntu and tried to put windows back in its original partitioned space, fail, as apparently you ALSO need 128Mib free between partitions using GPT for OSX to be happy…
One more moving of the NTFS partition to give it that 128Mib space and still nothing… I don’t know whether I should bother the 3+ hours it takes to move the NTFS partition to give it more than 128Mib space see if that helps but I’m at the point of futility :/
I have a feeling that I’m going to require a full re-install of all 3 os’s because apple love their quirky one size fits one approach to installers and the boot process.
The worst thing about it is that it took me days of playing around to get a triple boot working, now I’m probably going to have to do it all over again…
So to finish off this little post of futility and arguing with apples weird disk utility I add;
Dear Steve Jobs,
Please can you fix it for me to be able to install a triple boot with osx, windows and linux without me having to bend over backwards. Essentially get your installer team a clue and some error messages that can help diagnose and repair faults like this without me having to backup all of my data from my windows install and my mac osx install (which I really don’t know if I actually have enough space for).
Thankfully in this instance ubuntu was a fresh, unused install, so that wasn’t so hard to nuke and carry on.
kthanksbye
Update: If anyone else gets this problem you will have to ensure that there is LARGER than 128Mib of space between your Linux/Windows partitions and your OSX one before you can proceed. I’ve now managed to get the upgrade done 
Thou shalt have no other operating systems before me.
Quite seriously, just install VirtualBox on the Mac and put the Windows and Linux into the virtualized computers…
I had a similar problem when I went to upgrade to Snow Leopard—on a MacBook Pro with a straight Leopard install—and Iran into a similar problem. I’ll be damned if I can remember how I resolved it, though. =/
Did you use Apple’s Boot Camp utility to setup the multiboot or did you re-partition on your own?
-jef
Even through there are caveats, I couldn’t imagine using any other approach than vanilla OSX install + other operating systems on virtual images. It’s so much less painful to maintain.
I agree with the VM people. Snow Leopard + VMs works a treat, and the GL passthrough is pretty awesome. Alas some parts of Clutter still break it with acceleration turned on.