MIMO card with NON MIMO Router

I am trying to connect a pc with an Edimax MIMO PCI adapter, to a 3COM Router.
There is some distance between them, but I get ~15-25% signal.
I see the SSID, try to connect, but I only succeed once every ~10 attempts.
when connection is successful, I stil can't browse the web properly.

A new laptop sitting right next to the pc, receives about the same signal strength, but connects with no problems.

I tried changing the adapter to a D-link USB G adapter, but with same results.
What could be the cause for the problem?