USB Drivers

Help!

I have an IBM ThinkCentre desktop running on WinXP. A few days ago, it stopped recognising all USB devices. I had a scanner and a printer connected to the usb ports and aal of a sudden, I couldn’t print or use the scanner. I tried to reinstall the printer but the message comes up that the printer is not connected or switched on. I checked the device manager and found that the USB is not isntalled. I reinstalled the operating by runnning reinstallation cd but nothing. I then tried to reinstall the chiset drivers and still nothing. I have checked in the System Configuration Utility for I/O devices, the USB is enabled. What could be the problem? What should I do to reinstall the USB drivers?