No, just install windows on another hard drive. Dual boot windows, when you start up your computer you'll be given the option 32 bit or 64. There are easier ways to going about this which I cannot mention here.
After some quick googling it appears there are programs that might help you with this.
http://www.cadtutor.net/forum/showth...ista-64-bit-OS
Apparently, while I know nothing about AutoCad, this site might help you as well
www.longbowsoftware.com
If none of this works, send me a pm.