Trying to buy a particular game, but having problems in the purchase window. The fields where i put in the month and year for my credit card are not responding, no matter how i click on them. Please help.

What browser are you trying this on? Just download Mozilla/Chrome/Opera and see if it works on any of them.

Browser version and operating system version?

I'm using internet explorer and windows 7

That might be your problem :) Try Firefox or Opera.

If the alternate browser doesn't work (it should), you should contact GOG support through the normal channels. This forum is mostly for community support and if the problem is not actually on your end, we can't really do anything to help you fix it. GOG is going to need to get their people involved and the best way to do that is to submit a support ticket. However, it should be noted that GOG does not work weekends, so the earliest you might get a response is sometime on Monday.

Internet explorer I'm amazed you got THAT FAR seriously I use opera 12 it's excellent and has speed dial windows which I use extensively for browsing similar to gog.com's game shelf

Have you tried refreshing (F5)?. I've experienced similar problems when the page hasn't loaded completely, usually because of slow internet connection in my case. Seems to be caused by the page being built dynamically and the controls being disabled while values are added (or something).

Do you have any addons on it?

Although, I do recommend you ditch IE and try another browser