What?  Vista's bootloader can now load Linux?!  Wow.

Anyways, skimming over the thread, you should partition and format the drives before installing either OS.  My suggestion for splitting is this though - since you're just trialing out Ubuntu, just keep it on a small partition.  Generally 8GB should be enough.  Then stick Windows on its own partition, for XP give about 8-10GB (more than enough) or Vista, 12-16GB.  I typically suggest a backup Windows partition, 6GB. Then partition the rest of the drive however you like.  The reason for this is that you can easily change OSes if the case arises - for example, if you want to install another Linux distro and get rid of Ubuntu, then all you have to do is format the 8GB partition and install - your other files wouldn't be stored on the 8GB partition so you don't have to worry about it.

Anyways, yeah, format the drives beforehand, as not doing so can stuff up the Windows bootloader after installing Linux (the partition numbers change).  If you know how to use the Windows and Linux recovery features, you shouldn't really have a problem though.

ok, ill partition everything properly before i start inctalling.
thanks for the help ZiNgA, tomokatsu and draggy!! :D