Try booting from the XP CD - that should let you enter the Recovery Console and give chkdsk /f /r a try.
Try a bootable diagnostic utility from the manufacturer of the hard drive. Here's some links to the different manufacturers: http://www.tacktech.com/display.cfm?ttid=287
Try this bootable memory tester: http://www.memtest86.com/
From the situation that you describe it's likely that you've had a power surge to the system - and that can cause many system problems until you isolate the damaged components.
The Unmountable Boot Volume is a failure of the system to mount (access) the boot drive. So, it can't boot. This can be caused by file system problems, physical problems with the hard drive, or problems with other hardware that affect the connections that the hard drive uses. If the diagnostic isn't obvious from the tests above, then you'd have to start with stripping the system down and testing the barest of components. If it doesn't Blue Screen, then it's in one of the components that you removed (and you can isolate that by plugging them in one-by-one). If it still Blue Screens then, then it's in one of the components that's left on the system - so it's time to replace them (one-by-one) in order to figure out which one it is. Here's a link to my suggested system stripdown procedures: http://www.bleepingcomputer.com/forums/t/70216/hardware-troubleshooting-via-system-stripdown/
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