Hrm.
I've diagnosed that, for one reason or another, the partition identifier byte of my Windows XP drive has been changed and I can no longer boot into it.
(http://www.armabot.net/pub/cpumgmnt.jpg)
My Windows XP partition is (D:), Disk 2 Partition 0.
I know that the physical drive still works because my computer boots, and as you can see, Disk 2 Partition 1 is the active partition, which boots GRUB (Linux loader).
So... I need to know how to change the ID byte back to NTFS without formatting the data.
It's not critical, but I'd rather not reinstall and reconfigure EVERYTHING.
The problem was that the NTFS boot sector had become corrupt one way or another.
I was able to fix this tonight by following the Microsoft KB article located here:
http://support.microsoft.com/default.aspx?scid=kb;en-us;153973 (http://support.microsoft.com/default.aspx?scid=kb;en-us;153973)
Also, I had to download the NT 4.0 Resource Kit, to get the dskprobe.exe (Disk Probe) utility. It helped me restore from the backup boot sector.
Hope this helps someone down the line :)