Error upon opening

That's pretty much it, it stopped working today but was fine yesterday.

Any tips?

I'd recommend just reinstalling it but to save yourself some time, just copy your .dat file so you won't have to download as much once you have the new copy installed.

bump, any help besides reinstalling?

I'm going to ask the obvious here, do you have administrator rights? The one thing that really bothers me about your screenshot is that your .dat file is a media player file why? Also a 5 gig .dat file seems a bit too much for the .dat file's size. I strongly recommend reinstalling if you have admin rights.

I've seen that WMP icon for the .dat before. My assumption is, people try to open a .dat from some other source, use "open with..." and leave the default "remember this..." checked, thus setting an annoying file type association.

I agree: That certainly sounds like a lack of Admin rights.

And .dat bloat.