Problem with USB 2.0
On various (early?) T40 models, USB 2.0 may not function. Symptoms include inability to connect to USB 2.0 devices at USB 2.0 speeds, the "This device can perform faster" pop-up in Windows XP, device ID assignment error messages from the Linux kernel, and frequent reboots of the USB bus and connected devices.
This seems to be a hardware problem with the USB controller on Intel's chip or related electronics. Sending the laptop in for service may help.
If the Linux kernel refuses to register the USB device, "modprobe -r ehci_hcd" may help. This disables USB 2.0 functionality for the session.