Boot the computer. With it running plug the external keyboard in, you should see it installing software in the bottom right corner. Then a message that your hardware is installed. If you don't get that, try plugging in another USB device into that port. If that works then the keyboard might be the problem. Try it on another computer if available.
If other USB devices don't work, be sure USB is enabled in the BIOS, if it's and option under Integrated devices.
I have also seen those adapters cause issues.
Check device manager (Windows+Break)/Hardware tab/Device Manager assuming XP, and be sure after plugging in the keyboard there are no yellow question marks or red exclamation points.