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  | vl1rec2  | 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  | vl1rec3  | 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