I've been able to buy games in the past via Steam under Crossover Games, but today it doesn't seem to be working.
The only thing i've changed is installing Flash Player so as to get the little bit on the main page to appear (which i have since uninstalled in case that is causing the problem).
One thing to note is that the Steam main page is appearing as if i was looking at it from another web browser, where as normally it doesn't have the "reasons to get steam" and links bar at the top.
There was an auto-update to the client a couple of days ago if that could of messed it up?
edit - not sure if this will work but heres a pic to show what i mean by appearing like in a separate web browser
The "just another reason" ad and links at the top don't normally appear in the Steam interface.
edit - made picture a link, couldn't get it to work otherwise.