The disk management utility in Vista will happily let you shrink the Windows volume to free up space for another os...but be aware that making free space with Vista is easier than claiming that space back, should you decide to.

There are commercial partition tools out there, but I'm loathe to spend money to facilitate an experiment with free software. I've loaded Ubuntu into an extended partition, meaning that if I decide to get rid of Linux the "path of cheapest resistence" might be to convert the free space (where Linux was) into a virtual drive, rather than trying to expand my Windows volume (Vista alone might not let me do that anyway). It's no big deal, I've often used virtual drives in the past just to help keep things organized. There's freeware that will do this much for you.

Just be aware that the partitioning tools in Vista are convenient, but not particularly powerful. It's all moot of course, if you're willing to shell out maybe $70 or so for commercial partition management software (which you might only use once).