A month or so ago I tried to plug my Kingston USB pen into my PC (XP SP2) to download some photos. The Usb pen said it was being charged but the PC seems to have no inkling that it's connected. I can't force it to pick it up when it can't see anything there at all. I've not been able to connect it to the PC since.
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Also, try a different USB port. Windows, for whatever reason, sometimes decides to ignore things that are plugged in to certain ports (for me at least). If you have another computer handy, give that a try as well.