USB\ROOT_HUB20 listed as Unknown in Device Manager

Hi,

I am new to all of this so may not express the problem too well, however....

My PC(Dell Dimension 2400, running Windows XP SP2) has stopped recognising any USB devices. I have tried connecting an iPhone, an external hard drive, a flash drive and a digital camera.

I've read a few forums and after trying a few things have noticed that there is an Unknown Device shown in Device Manager. On looking further I found that it is USB\ROOT_HUB20 so I thought this may be the problem. I have tried deleting and restarting but it simply re-appears as unknown device.

Some one suggested I should "Re-install chipset drivers?" which I did, bt no joy its still listed as an unknown device.
I have also completed a BIOS upgrade but again to no affect.

I have also noticed that the only working USB device I have is a printer which has its own power source, so thought perhaps that could be the cause i.e. a lack of power to the other devices.

Can anyone help and is this possibly the cause of my problem with the above devices ?