Hi dan
i know nothing about this math subject, but i have tried it, i have MinGW installed, and as you said :"gcc matinv.c -O3 -o matinv", the results is:
elapsed time = 6.204 seconds
but running the main.txt(main.c) in the bottom of the previous page:
elapsed time = 11.031 seconds

i have pentium D cpu 3.0 GHz dual core but of the oldest versions( may be 5 years old or more).
if for some reasons the exe's different, here is my 2 exe's if someone want to try.
http://rapidshare.com/files/411145513/matinv.rar
(virus free, checked with avast)