available disk space displaying error? (df)

[dogmad <dogmad@poczta.onet.pl>]

I'm sure I do have a lot of free disk space, but my df output is:
Filesystem           1k-blocks      Used Available Use% Mounted on
/dev/hda7            -1848747039517         1         0  19% /
/dev/hda6            -1848747039517         1         0  31% /home
/dev/hda2            -527905053080         1         0  68% /mnt/hda2

from different OS i get:
/dev/hda2              2016044   1282844    630788  68% /
whitch seems to be correct.

I was able to repeat few times:
cat a >> b; cat b >> a,
so I really have space on my hda.

I've overriden df so that it printed (hartcoded) correct information,
but some installers (for ex. ZendStudio, and mysql-server 4.0)
still claimed that there is no enough room...

Anyone have any idea what's going on? (i know that '-' is from overflow, 
byt hda7 has 7GB and hda2 only 2GB...)