If you have a message saying the partition needs repair, run
chkdsk /f /r
You'll be asked if you want to run it at next reboot, since with those
parameters chkdsk cannot operate on the OS partition if the OS is
actually running. Answer Yes to the question.
Then shutdown, and reboot. Chkdsk should run fine if the hardware's OK.
When the OS reappears, then try defrag again.
Note that the built-in W2k defragger may not run successfully if there
is insufficient free space in the partition. Should that be the case,
you'll have to delete and/or move some files to another partition, or
use a 3rd party defragger that doesn't need as much free space. There
are several good, powerful such defraggers.
