If you have free, unpartitioned space remaining on your hard drive, you can simply use your Windows CD to do it...it's free, fairly easy, and something you already have. If you want to split one partition into two, Partition Magic and Paragon Partition Manager both have shareware versions. However, since these are trial versions there are severe limitations...Partition Manager, for instance, doesn't allow you to apply the changes until you buy it. Also note that there is some risk inherent in splitting a partition which could result in the loss of data, so be sure you have a recent set of backups, just in case.

Hope this helps,