In some cases GRUB will be broken when you reinstall Windows. You can fix it using an Ubuntu liveCD or another program, but these usually take quite a long time to boot up. I found a tutorial online that allows a user to rip the GRUB from an Ubuntu CD. I now use this CD to fix my GRUB. It boots up almost instantly. The live CD that I used is Ubuntu 7.04. This may be able to fix the GRUB error 15.
In order to run it, you must boot off the CD and/or DVD drive.
I do not remember where I got the tutorial to make the live CD, but the following link may help.
I made a simple tutorial to use my GRUB liveCD. This is in command line mode, therefore, you will have to type in the commands. It is short, simple, but best of all fast.
These commands work in pretty much any Linux system. You can use a liveCD if you want to.
If this does not work, you may have a more serious problem. You will have to look at http://aprivatebeach.com/blog/a-secure-pc/#Ubuntu_error_message_grub-install_(hd0)_failed