Shit. I think I just destroyed my harddrive. I got bored, so I once again put in my Ubuntu Live CD. Fooled around on Chess for a bit, then logged off, ejected the disk and restarted, aiming at getting back to windows. On restarting, I got a Grub Error. (21, I think.) Windows didn't start. I put the boot disk for 'Buntu Back in to check out the harddrive. I clicked it (Both the parts, it's partitioned.) The Former C drive "Cannot Be mounted" but the D drive is functional. (All the file s show up.) Help!

That is bizarre. So you tried an Ubuntu live CD, but now you get a GRUB error? That means that GRUB installed itself over the Windows bootlodaer. You sure you didn't install Ubuntu? Fear not as your data is probably fine. You just need to fix the bootloader. You can either use Ubuntu's grub-install, or use the Windows recovery cd. You can run fixboot and fixmbr

Well, I've got Ubuntu on another drive (Not a partition, another physical disk) But that disk is unplugged on my desk. Windows recovery CD, hm? I'll look around, see if I've still got it. Anyway, what about my drive being unmounted? Will the Windows bootloader thingy fix that?

Well that's why. You need to either a) tell your motherboard to boot off of the Windows hard drive, or b) plug back in the Ubuntu on the other drive because that has the bootloader on it.

Plug in the other drive? Ok, I'll try it. And... I've told it to boot off the windows drive, still gives me the grub issue. And... I can't find the windows XP disk, so I can't get the boot loader. Any other ideas?

AHH! I'm freaking out!!!
I've only got a few more hours to fix this!!

Doesn't the grub menu have a Windows entry. You could boot up to windows that way.

Also, reconnect the Ubuntu drive and boot up to Ubuntu. Look in the /boot directory. Is there a backup of the original MBR file?

If so you could copy it back to the MBR, but first, backup this file onto an external drive like a pendrive or floppy. Also backup the current MBR.
sudo dd if=/dev/sda of=/media/disk/mbr-backup count=1 bs=244
cp /boot/<mbr-backup-file> /media/disk

This assumes that your boot drive is /dev/sda, which is the first drive.
Also make sure you have a live distro disk handy. Just in case.

sudo dd if=/boot/<mbr-backup-file> /dev/sda

I got it to give me the option to boot windows, cool. But when I click it, it says NTLDR is compressed and brings me back to the boot menu. I've got a backup of all my files, and the system files for windows are back. The drive is remounted.

Whoo! I got it, thanks.