Numerous users are reporting that the PC version of Epic's action game Gears of War was completely disabled yesterday. Upon launching the game players would receive the following message:

"You cannot run the game with modified executable code. Please reinstall the game."

It seems the problem occurred when the game's digital certificate expired. The expiration date on the certificate has been set for yesterday (Jan 28); as soon as the date switched over, the game became unplayable.

"This was a surprise to us too," said Epic Games programmer Joe Graf. "We have been notified of the issue and are working with Microsoft to get it resolved."

Apparently, such an issue was also noted recently in Turbine's Lord of the Rings Online. Similarly to LotR Online, users found a simple workaround for the Gears problem: "setting the system clock back to a date prior to the expiration."

Source.