defrag problem

I am running Windows 98SE. I am not able to defrag! This
has been going on for many months and my computer is
getting slower and slower.

When I start the defrag procedure, I get as far as the
window showing the drive I want to defrag, I click "start"
and then everything freezes.

I have to crash the computer and re-start it. Sometimes I
get a blue scan screen since it was "not shut down"
properly. Sometimes it starts without this scan.

Please help me!