odd problem with external usb hard drive

I have been having an odd problem with external usb hard drives. Since upgrading to a new computer, my external usb hard drive no longer works properly. My new system is a pentium 4d 950 running on an asus p5nd2-sli. 1Gy ddr2 ram. running windows xp sp2. This issue is that I have a 160 Gb ultra ide hard drive that I put in a usb 2 external enclosure. But, everytime I try to transfer files over, it eventually slows to a crawl and ties the whole systme down. IT never finishes transfering. The whole desktop becomes unresponsive but if I turn off the external drive, the desktop immediately snaps back to life. I have exchanged the external enclusre and tried this with my original seagate 160Gb drive and a new maxtor 200Gb that I just bought, thiking it was the drive that was defective. Proper usb drivers for the motherboard are installed on the system already. Any idea how I can fix this?

Thanks for you help!