Windows Cannot Setup HomeGroup on This Computer error

I have two computers running Windows 7. I need to set up the homeGroup.
But My Dell Dimension 4700 displays the error message "Windows Cannot Setup HomeGroup on This Computer". Then I run the HomeGroup and the networking troubleshooter.
I had setted file and printer sharing on, and share off password protected . And I adjusted both machinge with the settings. It still can not.....

I updated the firmware in my D-Link router. but that killed it. reset would not work so $130 later my new Linksys router solved the problem.
The problem turned out to be the router.

The HomeGroup was joined immediately.