Some people prefer to have Linux loaded first, then
The LILO boot file is more stable that Wins.
(word of caution, LILO is a pain to remove. you'll need a
disk wipe utility to clean it out)
I'm not sure where to DL it from, I just bought RED HATs
home edition for like 20 bucks. I wanted a "mainstream"
linux system so I went with that.
But it's out there....
Sorry i'm no help with the Linux on top of WinXP.
Depends on what type of Installation you want and what distrobution you have. I guess a partition of 2 to 3 GB would be well enough. I have Slackware 8.1 and I did a complete installation on a 3 GB partition which now has 1 GB left.