I would avoid it if you can. The Vista version is nearing completion and I would be surprised if you couldn't use it by Christmas (if not by the end of October). You could also look at a dual-boot system instead. i.e., installing both Vista and XP such that you can choose which loads when you boot the computer. Running XP natively will certainly provide a better environment for DirectRT than would running it emulated through Virtual PC. It would work, it just wouldn't be ideal.