There could be a number of reasons XP won't shut down, but the most common is that ACPI is not enabled. Check this:
Start, Control Panel, Performance and Maintenance, Power Options Tab
Then click APM, Enable Advanced Power Management Support
And yeah, if you can instantly render your motherboard lifeless if you botch a BIOS update. Updating the BIOS is fairly straight forward, but if you don't follow the instructions to the LETTER, you can easily end up very, very disappointed.