We started migrating from Office 2007 to Office 2010 recently and came across a peculiar issue of not being able to change the existing signatures, add stationary or change fonts etc.
After loads of googling, we came across this post
and one of the comments found mentioning about 64Bit version of Office 2010
We confirm the method mentioned solved the issues
The instructions provided there with the slipstick post is quite self explanatory, please follow the post carefully to solve the nasty issue