In reply to your E-Mail, Only thing you can check is look in BIOS as the computer starts, You may have to keep tapping Delete key or F1 or F2 on some it's F10. Check to see if BIOS can see the drive, if it can you may start with the XP cd then run Repair BIOSas to be set to boot from CD as first boot device Hard Drive second boot save before Exit
A boot sector virus.....If you can get hold of a Media centre CD, set your BIOS and boot from it...If this works, post back with for instructions on using BootRec to recover the MBR on your hard drive.
If not, does your machine have a recovery partition? If it was not supplied with a CD or DVD then this is likely, You need to access it by pressing either F2 Or a funtion key on the keyboard. These partitions can restore your factory default settings...BUT.....you will lose data using this method.