run steam in windows xp service pack 3 compatibility mode and as an admin
thats what got it to stop doing that bullshit
 
If you want tech support, you kinda have to provide more details than the fact that it wasn't working earlier, and it still isn't working later.

What has changed since the last time it's worked? Has it ever worked? What version of Windows are you running?
 
Macs aren't the best computers for gaming, you know.
I ran TF2 on a Macbook Pro and it ran fine. It's not really Mac vs. PC, it's OpenGL vs. DirectX and OpenGL isn't as good for a lot of rendering although it's getting much better recently. I would try and delete the ClientRegistry.blob and see what happens.