@hazardwarning Windows menu>right click on Computer>properties to see if you have service pack 2.
One thing you need to be aware of is, if you re-install Windows (in order to get it into a smaller, cleaner state) onto a machine with a Linux partion it may overwrite the GRUB bootloader and only leave you with a Windows boot option. I'm not sure of the best way around that. @matigo might know.
You could skip the re-installing bit but if Windows is currently huge your Windows VM is going to be huge too.