You could maybe look into Virtual PC (depending on what you're doing with the 2 OSes). It makes it so you don't have to reboot the machine to start up a new computer in your computer. Pretty hard to explain.... But if you aren't doing anything too demanding with your machine, and you have half a gig (at least) of RAM then you may want to check that out. And you can make it so your 'virtual pc' has a hd that expands as it needs. And on your regular PC its just a file. Pretty slick program.
http://www.microsoft.com/windows/virtualpc/default.mspx
(Yes, microsoft, but it really is a cool product)