I recently tried to update my Operating system from Xubuntu 8.10 to 9.04 and for some reason whenever i tried to boot the computer all i could get was: Grub Boot Error 15. How about writing a tutorial for Grub Boot Error 15? No matter where or who i turned to for help i couldn't resolved the problem and i was very fortunate in my opinion that i was able to re-install Xubuntu 8.10 ortherwise i wouldn't have been able to get my computer up and running again.

Grub Error Code 15: "Specified" File not Found

sounds like your upgrade went astray somewhere. Either it changed the file name in Grub and not the actual file name or Vice versa. In which case reinstalling from scratch may have been your best (or only) option.

Here is the GRUB manual, numbered codes are addressed in section 14.3 http://www.gnu.org/software/grub/manual/grub.html

I'll look into writing a GRUB Tutorial but the big issue I see with doing any Linux tutorial is that things change from one distro to the next so you never really get good coverage of an issue.