Here's a paragraph from our Knowledge Base:
"This error may occur if the 64 bit version of WaterGEMS/WaterCAD/HAMMER has been installed along with a conflicting version of Microsoft Office. When installing on a 64-bit operating system, the program should detect the presence of the 32-bit version of Microsoft Office and force the software to install as 32-bit (since the 64-bit Access Database Engine component used by WaterGEMS/WaterCAD/HAMMER conflicts with the 32-bit version used by the 32-bit version of Office). In some rare cases, 32-bit Office is not detected and the software continues installing the 64-bit version, which may lead to the above errors."
Link to our KB: http://appsnet.bentley.com/kbase/
In the link above if you search for 65638, you will find the above paragraph and some other details.
Note: I found that Internal Explorer works best for the KB. So, please use IE.
Thanks