RIM supports installing BES on a SBS up to 15 users. After that its recommended that you move the BES software to another machine.
Windows XP is not a supported operating system. Good luck trying to get support for that, first of all it will be a pain to install and then no consultant or support rep is going to talk to you if you have windows XP or virtual machine software running.
Virtual Machine software is not supported for BlackBerry Enterprise Server installation
Last Updated: 15 September 2005
Article Number: KB-04405