I have a Packard Bell desktop with XP and W7 installed. When I switch on the (BIOS?) menu appears with both drives selectable in this order. If I do nothing XP fires up by default. I would like now to switch the default to W7. My PC shop tells me that a special program is required to do this whereas I had thought that I could do this somehow via the BIOS settings (just how is not clear). Is this indeed a complex task or can I DIY?