I have 21.5GB free. Why won't my MacBook Air 2010 let me use Boot Camp?
To expand upon my above comment:

64GB is just not very much space, especially when you consider updates, your own programs, data, etc etc.

My recommendation, if it is possible and available, is to run Windows XP from a USB drive. There are a handful of documented procedures for getting WinXP on a flash drive and running it from there. I would recommend following this TechRepublic article to get it all set up for that.

This should negate the issue with Boot Camp, let you keep the 64GB of HDD space to your trusty OS X partition, and hopefully provide enough room on your flash drive for WinXP to grow with applications as well. 

Hopefully there is nothing Vista or Win 7 specific that you need to do. Good luck!