This may be a bit of a fly/sledgehammer suggestion, but...
Do you have a spare hard drive (or partition, if you know what you are doing) that you could use to try a separate install of Windows on? If the game worked fine previously and you have not made any hardware changes, then it seems likely some driver update, anti-virus program update or other software install is causing a problem. A clean install of Windows may be easier than trying to remember every change in the last 2 months and systematically undoing them to check it it helps the game, but I wouldn't want to have to redo everything (program re-installs, settings changes, etc) on the main Windows install only to find the problem was something else.
