you will not find too many engineers using macs...less than 1% I bet.
A good laptop will run autocad 2007, it doesnt have to be a mac.
Autocad 2008 will run on Vista - I take back my last comment.
Laptops are overrated anyway, if she's doing heavy CAD work she should have a desktop with a big monitor - unless of course she buys all the peripherals and has it act as a workstation...