XP can't do that. There are some excellent programs that can but they all cost around $40. I use Partition Magic by Power Quest. It's safe and effective.
There are some freeware ones that also do that but they require a fairly high level of understanding or else the data will be lost. They're not for the faint of heart.
Among them are Ranish Partition Manager that can be found with any search engine. It works well but requires lots of reading. It makes the $40 stuff look like a bargain.