FASTFAT is a kernel-level driver that provides support for FAT16 volumes in Windows 2000, including features like delayed writes that are normally only supported for NTFS.
Sometimes a Windows 2000 system will crash or refuse to start, citing an error in the file FASTFAT.SYS. This may lead an administrator to believe that FASTFAT.SYS is corrupt, which can happen even if there are no FAT16 volumes mounted. (FASTFAT is also for reading and writing floppies, which is part of why it loads by default.)
However, the first and more likely culprit, interestingly enough, is not the driver itself but a faulty SIMM or DIMM in the computer. If a FASTFAT error turns up in a system that has not had a problem before, the memory may be the first culprit, and you should swap it out to check. Some flavors of memory, from specific manufacturers, do not work well in certain brands of motherboard. With DIMMs, one easy way to determine this is by running the machine first with a single DIMM and then with a pair (or more). If the machine runs well on one DIMM but badly with two or more, then that manufacturer's RAM is not working with this particular machine and should be changed out for another variety. (In the event that a machine refused to boot before now boots properly with a change in memory, the memory is the number one culprit.)
Barring faulty RAM, the driver itself is probably corrupt, and will need to be replaced either through a parallel install of Windows 2000 or via the Recovery Console. FASTFAT.SYS exists in the %SystemRoom%System32Drivers folder of the operating system, and is stored as FASTFAT.SY_ on the Windows 2000 CD-ROM. Running a repair operation from the CD may not work; the file should be replaced by hand.
Serdar Yegulalp is the editor of the Windows 2000 Power Users Newsletter. Check out his Windows 2000 blog for his latest advice and musings on the world of Windows network administrators -- please share your thoughts as well!