If you are using Windows XP (although I think that Win7 suffers from the same problem as well), it has a nasty habit to cache USB drivers for fast startup when you connect a device to a specific USB port, which in turn may lead that once something is f-ed up with that driver it will always load the faulty one from its fast-run cache, thus effectively bricking the device support or even failing to recognize it. I had many pains with my N95 until I found out what was causing it. Since N900 charges through the USB, if it fails to negotiate with the system it won't get its 500mA but only 100mA USB current in which case N900 will report that the power is too low to charge.

The simplest way is to remove all device-type drivers installed on the system (they will reinstall themselves when you connect your device again). To do so, go to System Properties (Win + Break), Advanced tab, click on Environment Variables, and add a new system environment variable devmgr_show_nonpresent_devices and for its value set 1. Go back to the Hardware tab in System Properties, go to Device Manager, find all Nokia drivers (and possibly some other USB devices that you no longer use) and remove them...

Now when you connect your device it should reinstall its correct drivers and resume its normal function.