Ah, but that's just a mod of SE IV. You can have the original for FREE now. GO to:
http://sc2.sourceforge.net/index.html
This is home of the FREE (and open source) Version.
The source only became available a few months ago, as I recall, so they're not very far along but it's in the hands of the gaming 'public' now so it's only going to get better.
If ONLY we could get the source to Master of Magic released!