Make sure you only have Acrobat 9 installed. Your system may not have completely removed version 8, which will cause Acrobat to crash. This would happen repeatedly here in another situation also, with only Acrobat 8 installed and Reader 9 installed on the same computer.

I have managed to go from CS3 to 5 without any problems. Can you backsave your CS4 docs to CS3 and try that?

In the “old days,” if you tried to save a file made on a trial version of something – altho' not necessarily Adobe products – , it would crash. Could be a permissions problem.

And as simple as this sounds, corrupted fonts have been known to do wierd things like this.