Usb not recognisable

Hello,
my pc is not recognizing or showing memory card reader. in the device manager under usb controller it showing unknown devices. and showing there is no device driver details available on clicking the drivers details. my os is windows 7 and motherboard is of asus P5G41T-M LX. i have unistall unknown devices and scanned for hardware changes but nothing helped. memory card is ok as i have tested them on other computer. plz help