I have Absynth 4 and 5 installed on my Win7x64 machine.  I can insert an instance of Ab4 fine, but inserting Ab5 causes a system hang.  Any ideas?

If Absynth5 is running at the VST3 standard it wont load correctly as the VST libraries are loaded to the program data folder on the main drive. The UI interface will load in some cases however it will cause problems. Project 5.2 being unsupported only will run VST 2.X. If it's not VST 3 I don't know what else to tell you. I suggest upgrading to X1 as it may run this plugin better or continue to use Absynth4. Hopefully you can find a workable solution.

Fortunately it seems a rescan fixed the issue.

It's great that it was as simple as that, I know that when you have Sonar and Project 5 they sometimes change the registered VST's and a rescan is necessary. Great that your able to get it to work.