|What was/were your problem(s) before he re-formatted the laptop??|
Only one partition on a hard drive can be active at any one time. XP's Setup would have done that for the C partition automatically.
There are only a few common reasons you would get that 'Operating System not found' message - the boot order settings in the bios are wrong, or some device in the boot order is not bootable, or the hard drive is failing.
I know from experience that you can run XP's Setup successfully even if the bios's boot order settings are wrong, but that's not likely when you have only one hard drive with only one Windows installation on it. The boot order settings should be floppy drive first, if you have one, CD drive second (or first if you have no floppy drive), hard drive third (or second if you have no floppy drive. The hard drive must be in the boot order settings in order for you to be able to boot from it.
You may need to unplug your removable drives when you boot.
Note that some boot order settings can be set to boot from removable drives first, or the bios does that automatically (e.g. USB flash drives, or USB or firewire connected external drives). If you have a non-bootable removable drive plugged in when you boot, some bioses will not try the next device in the boot order list to see if it is bootable if the first device is not bootable, and you get the 'Operating System not found' or similar message.
If you have a floppy drive connected, either conventional or USB connected, if it has a floppy disk in it that is not bootable, you will probably get the 'Operating System not found' or similar message.
If that's correct.....
If your hard drive is failing you probably had all sorts of problems before your friend re-formatted the C partition on the hard drive. Laptop hard drives are warrantied for no more than 3 years, and they tend to fail sooner than desktop hard drives.
Check your hard drive with the manufacturer's diagnostics.
See the latter part of response 1 in this:
(thanks to Dan Penny for this link:)
Hard Drive Diagnostics Tools and Utilities
If you don't know which brand of hard drive you have, you can often see the brand when you remove the cover to where the hard drive is, or Seagate's Seatools will test any brand of drive.
If you don't have a floppy drive, you can get a CD image diagnostic utility from most hard drive manufacturer's web sites, but obviously you would need to make a burned CD, preferably a CD-R for best compatibilty, on another computer if you need to.
If the hard drive itself tests okay, any data problems found can be fixed, one way or another.