What device is it?
Have you had this device working before or is this the first time you have connected it. It may be that you need to install something off of a disc before you can use it.
If none of the above applies then try this,
I'm not sure of the exact procedure as I am on Vista, but I think you do this,
Right-click My Computer, click Manage, and then click Device Manager.
Once in the device manager you need to look down the list for "UNIVERSAL SERIAL BUS CONTROLLERS" click the + next to this, you need to right click on each of the entries that have opened up after you had clicked the + and choose from the menu that appears UN INSTALL, after you have done this for each of the entries, restart your PC and it should then detect and reinstall the USB CONTROLLERS, once it has COMPLETELY FINISHED doing this, try connecting up your device and see if it works.