When I downloaded 3.8, for some reason when I opened replays, they opened in 3.72.

So I deleted 3.72 as I had 3.8 and told the replays to "open with" toribash 3.8

But everytime I try and open the replays it comes up that the replay is not a valid win32 application.

Anybody know how to fix this?

Try deleting yuor old 3.8, restarting your PC then reinstalling your 3.8 and then restarting again.

That might help.