Before you do this get a copy of something like Partition Magic if you don’t get it just right. I did this before I finally upgraded to Vista and frankly, I didn’t want to be bothered with all the aggravation. Ubuntu can be downloaded and it is easy enough, but the bootup menu can be troublesome and may lock up your computer as it did mine. I finally got a work around off the internet which I installed but the truth is that I didn’t see the point of it. As I understand it only about 1% of the PC population uses Linux.
Good luck. You will need it.
The main thing I dislike about dual booting is that it means I have to spend a few minutes to switch, and that I can't be doing two things at once, on both the Windows boot and the Linux boot.
My preferred setup now is either Linux (for myself, a Linux kernel hacker) or Mac OS X (for my wife, who just likes things to work), while running Windows in a virtual machine (VMware on Linux, or Parallels on Mac OS X). We only run in Windows what we have to, such as, in our case, Quicken and Tax Cut.
But such virtual machine setups cost extra money for the better virtual machine software (VMware or Parallels), and really are best done on systems that have enough RAM memory to hold both operating environments in memory at the same time -- twice as much memory as you need for either O.S. by itself.
The only time I fire up a dual boot system into a native Windows boot is to run a game for a few hours of escapism.
Another reason to give Ubuntu a try - they have the largest, most active forum. There’s a section just for newbies. This is so helpful when you’re full of questions!