You need to get a USB-RS232 converter which works well in Windows 7 x64 PC. Thats all you need. Then in VM configuration (turn off VM) add a new serial port (if you have printer enabled by default remove it as it may get conflicted with serial port.). In serial port details select it to choose the new COM port which is listed after adding USB-RS232 converter.
If you have added serial port as Serial Port 1 in VM, then you can access serial port in Kubuntu 10.04.1 as /dev/ttyS1