Assuming Windows XP, go to the control panel, system, hardware, device manager, click on the plus sign beside the listing for ports, right-click on your usb-serial adapter and select properties. In the port settings tab, make sure it's set for 9600,8,none,1, no flow control. Click the "advanced" button on that tab and make sure that the port number is no higher than 4 You can change the port number there - the B&K software stops scanning at port 4 (I think.)

If you still can't connect, try unplugging the CT for a few minutes.

I've found that the communication on the CT's can get a little flaky if it's even moderately warm. (In fact, the CT on the whole is pretty heat sensitive - great machines, but they don't like the heat.)

(edited - sorry, I didn't see your reply about the com port being correct. It came up while I was typing my reply. I type slow!)