I recommend Ubuntu or a distro based on Ubuntu (PCLinuxOS and Linux Mint are very friendly).
Usually, when installing Linux, it changes the Master Boot Record (MBR) of the default boot-partition (I assume either Vista or XP). It then adds entries to a boot-menu (it uses either LILO or GRUB) so that you can choose either XP, Vista or Linux. This works well enough, except for 1 small niggle: if you should remove the Linux HDD, or wipe that disk, then you'd need to restore the MBR using your XP or Vista disk, so that it boots as before.
If you want to keep the 2 Windows HDD's unchanged and be able to select either of those or Linux from a boot-menu, you'll need to do some fiddling.
- Remove the 2 Windows disks (for now)
- Change the boot-configs so that your PC boots from CD
- Install Linux on the 3rd disk
- Once that's working, it should present the Grub (or Lilo) boot menu and you can log into Linux.
- Shut down the PC and reconnect the 2 Windows disks and boot into Linux (you won't have menu options for Windows yet)
- Edit the file /boot/grub/menu.lst in Linux, adding entries for the XP and Vista OS's. This can be a slightly confusing step, since you need to know which devices and partitions contain those OS's. I can post my menu.lst file this afternoon from home.
Ensure that you backup the default file!
There are many guides for identifying the correct partition info in Linux, just do a little Googling
