All drive letters assigned to no drives

I have several PCs with Windows 98 that have recently had an odd problem occur. In My Computer, all drive letters A through Z are listed as drives. The letters assigned to actual drives are all correct, but the letters that are essentially assigned to nothing indicate 0 MB in their respective properties windows. I cannot map a drive either because I am told that there are no available drive letter available.

Any help would be greatly appreciated.