I am getting the same error message for a few days now (it worked before for over a month on the same laptop) and I believe it is related with the Nokia Phone Updater software which updated to the last release and replaced some of the drivers the day this started to happen.

This is what I observe:
1) I plug the USB cable to the very same port I had earlier and the phone will start charging.
2) The phone will charge over USB until I select one of the connection methods (either Mass Storage or PC Suite), 1-2 seconds after I select the method
and option I get the error: "not charging. insufficient power"
3) I am using the original Nokia USB cable as I did before
4) My laptop uses a docking station and this happens disregarding of using the power and the laptop battery charging or being full
5) I checked the USB port and confirmed it has 500mA, I also disabled power savings on the port, and rebooted both computer and N900 but did not make a difference
6) Under Device Manager > Show Hidden Devices > Wireless Communication Devices > I see "Nokia N900 USB Phone Parent" showing a question mark indicating an error with the driver. I tried rolling back the driver, uninstalling all Nokia drivers, nokia software, rebooting, etc. But I still get the same error (important to select the option to Show hidden devices from Menu > View)