Have you done a windows-update the last time? I´ve had a similar problem in the past. Could happen after every windows-update to any component of your pc! (thank you Mr. Gates!!
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Microsoft doesn´t like it, but i removed the auto-update. So you can decide by yourself, what to install, and what not. Not all what comes from them is really neccesary and usefull (like SP2 for XP, what caused many problems)
I´ve handeled it with removing the component, shut down the PC, restart, new plug in (and then it doesn´t really matter, which USB slot is used)
Ralf