If you can get V19, get that and install both V18 and V19 on your Windows 7 machine.
V18 is Vista compatible, but why bother with vista???
If you can't get V19, might as well just stick with XP.
I currently have V15 - 19 on my Windows 7 machine.
EDIT: Or do like diat150 just said, Windows 7 with an XP virtual machine.