The official charger does not output 5V, it outputs a higher voltage than that. All devices with the tiny plug will reject 5V with "not charging". The cheap non-nokia usb-> tiny plug adapters are just wired straight with no boost cicuitry, and whether it will work or not will depend on how much higher voltage than 5V your computer's USB outputs, and how inaccurate the voltage sensor in your device is...

There's a document at forum.nokia documenting the voltage and current windows the device will accept. Basically there's a "Not charging" area at and around 5V. Voltages above that are identified as fast charger. Voltages below are identified as "special" charger, and the device will use a charging mode optimized for more stranger sources of power, like solar panels.