4.8 - / out of space

[McCy Ron]

System: FreeBSD 4.8 with standard config on PII/400...used mainly as a 
backup server.

df shows...

Filesystem  1K-blocks    Used   Avail Capacity  Mounted on
/dev/ad0s1a    128990  119660    -988   101%    /
/dev/ad0s1f    257998       4  237356     0%    /tmp
/dev/ad0s1g   7426528 2109420 4722986    31%    /usr
/dev/ad0s1e    257998   15888  221472     7%    /var
procfs              4       4       0   100%    /proc

My root file system is full and I can't account for why this is so. I 
used du on all of the directories on / and could only come up with 
28000K of usage - far short of what it's supposed to hold. The User 
Manual suggests that there my be some files not accounted for by du 
actually residing on the system. What is the best way to reclaim this space?

The computer still runs despite this file system being full so I'm no 
panicking YET.

Thanks