New record position

Hello,
I am using Slackware Linux 10, Postgresql 8.0.1.
My computer had a incorrectly power down (last week) and I have executed the
vacuum command:
VACCUM FULL ANALYZE VERBOSE;
to recicle and verify my database.
Before the power-down, all records had inserted into a table have displayed at
LAST record. Like:
SELECT * from tb1;
f1 | f2
------|--------
rec1 | vl1

INSERT into tb1 values ('rec2','vl2');
SELECT * from tb1;
f1 | f2
------|--------
rec1 | vl1
rec2 | vl2

But After the power-down and vacuum, the new records inserted have appeared in
random location (FIRST, between other records, etc...). Ie:

INSERT into tb1 values ('rec3','vl3');
SELECT * from tb1;
f1 | f2
------|--------
rec1 | vl1
rec3 | vl3 <<=====
rec2 | vl2

Why it? I can't undestand why the new record location was change. Shouldn't it
apper at the LAST record???
What need I do??
Thank you.
--------
Lucas Vendramin
Brazil