Merge partition hard drive

I have a hard drive that I have partition into two drives, one with Windows Vista and the other with Windows XP. Now I want to make a third so that I can add Windows 7, but I have to allocate some space from both the Vista and the XP drive. So now I have two empty partition and want to merge the empty partition into one. How do I do this? If you mention some software, I would prefer it to be free software if possible. Thank you.

I think I might have found an easy solution to this problem. This is what I found. Insted of allocating space from both XP and Vista, Just allocate the space you want from either XP or Vista and extent it to the other OS. Once you add more space, now you can allocate space again from the OS you just added space to. That is it.