USB Pendrives are not getting detected

Hi Experts,

There is a XP machine. On which USB pendrivers are not detected. We we try to install it macnually from device manager providing system32 path name. It tries to install and at the end displays "Access denied". 

Steps i did -

1. Gave full permission to USBSTOR.pnf and USBStor.INF file to every users group

2. There is No Domain GPO to block mass storage device option.  Even after disjoining from domain (in workgroup) with administrator login, The USB pendrive doesn't get detected.

3. I have also replaced the USBSTOR inf/pnf files from other XP working machine

Even performing above steps the USB is not detected. What more I need to do ?

Thanks