See S-040365 for some possible causes and troubleshooting ideas.
I had the same problem.
I was able to narrow it down to the "Revision" variable.
You probably have an "Increment Revision" in your workflow.
You need to remove all reference for the pro/e "DRW" in the custom properties of the "Revision" variable.
The problem only showed up when we upgraded to Windows 7/64bit. Windows XP/32 works fine.