Several users have been reporting this problem within the first week of OFP Gold's release. See this FAQ item.
If you are playing through the Red Hammer campaign using OFP: Resistance, you should be aware that upgrades to some A.I. routines are not compatible with the sixth Red Hammer mission, Defend. This will cause Operation Flashpoint to crash to desktop (CTD) when starting this mission.
To resolve this problem with the Defend mission. run Red Hammer with the old OFP version's program file, OperationFlashpoint.exe. Make sure you have the original OFP version CD - not the Resistance CD - in your CD-ROM drive. You should now be able to play and complete the Red Hammer's mission without problems.