Can I remote desktop from Windows 7 64bit Pro to Windows XP 32bit Pro?
Yes, but only Professional edition of XP and above.
You have to enable Remote Desktop through
From there you should be set to go.
If you need any extra help, Microsoft have an article about it here.
Yes, you may get a warning. You can safely ignore this. Don't be scared to try it. Still operates on same terminal services. Vista/7 just have newer versions and seem to 'talk' better, but XP x86 Pro will still work as a host.