Halo Combat Evolved Error

I have been using cross over to run games
but i couldn't get many games
i am looking for halo to play online multiplayer game on mac..
Help me with it...
I have downloaded it and when i click run i get this error message...

Error:
Cannot allocate required memory. Some other application is loaded where halo needs to be allocated.

Help me i want to play multiplayer online halo combat evolved in mac book pro
using Lepoard os..

Give me step by step procedure to do so...
If possible give me screenshots..