PC adapter with usb-RS232 cable

Dear sir or miss,
I have a notebook(IBM T43) use winXP-p, since it has no com port, I use a USB-RS232 cable to connect pc-adapter (6ES7 972-0CA23-0XA0) and connect it to MPI port of the S7-400 PLC,but everytime I try to connect the PLC, the notebook dosn't work anylong. I have seen the config of PC-PG interface,and it seem has no problem. Someone tell me use the USB-RS232 cable would have a lot of problems, can anybody kindly tell me how can I solved the problem easily?