It's because the game's a bit old, and reads the free space on your disk improperly. You can edit the free_space parameter in the fallout.cfg to 0, that should solve things.
Zdroj: http://www.gog.com/en/forum/Fallout_Series/not_eno ugh_free_hard_disk_space/perm=4/
EDIT: A tady je zdroj z brahminy: http://forum.madbrahmin.cz/viewtopic.php?f=6&t=419 &start=15