First, as Charles says, check the ground connection. You should use the pin number 5 for 9-pins connector or pin numer 7 for 25-pins connector (it is signal ground). To get the ground from the outside of the connector is not a good idea. After this use the oscilloscope to check the power supply of your microcontroller circuit. I am sure, that you're getting some noises frome switching power supply of your PC. Use some electrolytic or ceramic capacitors to reduce them.

Jacek