Microsoft's own tools for converting live machines to VMs are really not very good from what I've seen. They're clunky to use and require that you go through a multi-stage conversion process which interrupts work on the original server. That said, they're free -- and if you don't have much of a budget to work with, they may be the only game in town. I've looked at some other migration solutions, such as LeoStream P>V Direct 3.0 and PlateSpin PowerConvert, which are a lot more seamless -- but also that much more costly, and typically charge per system converted. If you can spend the money to use such a tool, look into them.