So if I were to install a freeware OS...

[RomanClock]

What should I use? I hear Ubuntu seems like a good choice, though I have no experience with Linux based systems so it would be helpful if you could provide links on how to set something up like that (it would be a secondary OS in case Vista is mess up, I was even thinking of installing XP as well). It would also have to be 64 bit (if thats how Linux is set up, I don't know)
Right now I have a main hard drive (160GB 10KPRM) that has Vista 64.
Then I have a second hard drive (same as main) but I partitioned it into 90GB and 50GB sections and was thinking of using the 50GB one for another OS.

Any suggestions for a free OS?
Any suggestions or warnings about installing a secondary OS on a partitioned drive?