In a few instances this was due to the customer upgrading his Windows to service pack 3. Since Pro Tools 7.4 requires a security update, those who choose to let their computer automatically update may inadvertantly updated their OS ahead of some of their plug-ins. Also, some third-party plug-ins may not be compatible with Sp3.
The solution would be to either update the plug-in in question or uninstall Sp3.
Refer to Sweetcare article # 32319 on how to uninstall SP3 or get the missing update for Windows SP2.