After one of the Windows updates (back in September I believe), I haven't been able to run FlatOut. I would get the "please sign in with administrator privileges and try again" dialogue box. I waited a couple of months to see if GoG fixed anything, which they did according to the changelog. However, now I can't even install FlatOut as it gets to the end of the installation, stops, and reads: "broken installation".

Does anyone have any ideas? Thanks.

Just asking the obvious, but have you tried running the game as administrator?

edit:
Oh wait, was this the securom/safedisc problem in Windows 10?

Well now I can't even get the installation to complete, but yes when I was previously having trouble it was because of the safedisc stuff.

Apart from redownloading and try again, I can't think of anything better than to contact gog support.