It is always a good idea to make a backup before re-partitioning a hard drive. With that said, 99.5% of the time shrinking the Windows partition down and leavng the empty space blank and unformated works just fine. Just remember to manually add the Linux partitions so the Windows partition remains. if possible try and have about 10-gigs or more free for Linux, and remember the more the better.
You might find that a free partitioning tool like that found as part of the SystemRescueCD ( http://www.sysresccd.org/Main_Page
) is nice to have.