You can not officially install BPS on XP. See:
http://na.blackberry.com/eng/services/server/offers/professional_express.jsp#tab_tab_requirementsAs for other options you could vmware workstation, still need to install windows 2003 server on the vmware, then you could install BPS in the VM instance.
As for the WIFI, you will need a data plan for BPS, so wifi will not work in that situation.
http://www.vodafone.co.nz/business/blackberry/blackberry-enterprise-server-express.jspLet me know if you need anymore information.
Good luck!