User Admin in pgsql

Hello all,
After I Updatded my pgsql from 7.3 to 7.4.6 in debian with "aptitude", I
found that my admin user account's password has gone--everytime when I
login into  psql, I don't need type my password any more, and every
other user account too.
I know that a SQL command Alter could de used to alternate my password,
I tried it like this:" alter user xxxx with encrypted password
'xxxxxxx';", But it doesn't work at all.
What can I do?

BTW: My OS is GNU/Debian Sid 3.1, PostGreSQL 7.4.6 Now.
Thank you!
--
Wang Jun@ Atmos. Dep. of Nju Nanjing Jiangsu China.
GNU is Not Unix, BSD is Powerful,
Ada is Beautiful, Nju is Graceful.