usb cdc probleme
not sure what cdc means in this context but if you can access it via hyper terminal then your "usb" connection is nothing more than a com port emulator, so you are getting almost no benefit from having usb in the first place unless it is to link some legacy device to your new laptop with no rs232 ports.
Windows XP as an operating system comes in many flavours, service pack 1 and 2 for starters, VIA based motherboards and non via, the former you need to ensure you have the latest 4in1 drivers installed, the older hardware can have resrictions such as USB 1.1 only whereas the newer hardware has usb 2.0.
You should check that the old PC has all of the updates from windows installed and the latest motherboard system drivers also.
If none of that fix's it try using a seperate PCI usb card, very cheap off ebay and will give a seperate incoming line for usb.
If you old computer has (as I am sure it does) have RS232 9 pin sockets at the back , consider adding Tx/Rx/Max232 to your design to allow USB and serial.