When you disconnect a device from your Windows PC, it'll magically vanish from Device Manager -- out of sight, out of mind as it were. However, in the Registry, the entry for that gadget remains, in case you decide to attach it again. While leaving these entires isn't harmful, if you're a stickler for keeping your system as fresh as possible, you can use Device Cleanup Tool to get rid of redundant ones.
You can download the program from creator Uwe Sieber's website, which includes 32 and 64-bit executables. The presentation is kept simple, with a list of disconnected devices on the left and their "last used" time on the right.
You can easily remove one or several devices and the menu options allow you to create a System Restore point in case you break something.
As mentioned previously, removing entries won't speed your system up, but Windows is a complicated piece of software and this lightweight app is a nice-to-have in your diagnostic arsenal.