delete seems to be getting blocked

hi,
I am using postgresql 8.0.0.
i have four tables in my database
TAB1, has one primary key T1

TAB2 , has 2 fields, one is the primary ley T2 and the other one is the foreign key T1(from TAB1)

TAB3 also has 2 fields, one is the primary ley T3 and the other is the foreign key T2(from TAB2)

TAB4 has 2 fields again, primary key T4 and a foreign key T3(from TAB3)

the disk is 100% full.

i open psql <datbase_name> and do
delete from TAB1

nothing seems to be happening for a long time, although when i do top, it shows postgres taking 99%.

what is happening, cant delete be quickened?

thanks,
regards
Surabhi