I have recently been in conversation with a user who's been experiencing problems with EFLC if Carmageddon's installed. After supplying me with some error codes and event logs, it would seem that Carmageddon 126.96.36.199 is incompatible with current version of EFLC (retail. Unconfirmed for Steam version).
The error being produced is a simple crash to desktop once "Start" in clicked in the main menu of the game. The user who reported this problem had no problems running EFLC without Carmageddon installed.
I will look into this problem and update the handling files for both versions if I find the proper source of the problem.
This problem is currently unconfirmed for GTA IV.