No you cannot switch between the 2,
How do you know you r CAD software will not work on W7 64 bit, most 32 bit software works on W7 64 bit? Have you tried installing the CAD software and running it in XP compatibility mode. You do this by highlighting the exe file for the program and go the compatibility tab
What it means is you can install virtual machine known as VM Software. There are many out there MS has Virtual XP and it is free. when you run the virtual xp from MS you just load your programs on the XP machine like you would any other computer. There are some caveats though, with MS Virtual XP, The default for full screen resolution is 16 bit there is a regedit you can do on the XP Virtual machine that allows for 24 bit colour resolution