What version of SolidWorks are you running?
I've been using SolidWorks 2008 in a XP Pro x64 virtual machine for several weeks now without any issues. It's certainly not as fast as running natively, but its reliable and it gets the job done. In fact, I've yet to have SolidWorks 2008 crash once in my XP x64 virtual machine, something it did annoyingly often on my Vista Ultimate 32-bit partition (I blame Vista

).