A common cause of this problem is that you are installing a release that is not supported on Windows 7 or possibly a 64-bit OS. Which Mentor Graphics release are you installing?
Mentor Graphics Customer Support
I am running PADS 9.3 W7 64 bit. I have been installing this on W7 64bit for about 8 months with no problems, this just started showing up about a couple weeks ago, I am chekcign with my IT that handles windows and security patches to see if there was patch that caused this. I know that Mentor does not support older versions of Pads/DxDesigner on W7 but I have had it working.
You should check if there are existing Start menu entries and if so, remove them before you try again. Perhaps it is colliding with something that is already there.