postgres user password: Log on failure

Hello,

I'm on Windows XP with postgresql 8.4 installed by postgresql-8.4.4-1-windows.
exe with essentially default settings (only the path to the data dir was
changed from default).

I realized that sometimes (not always!) the server cannot start when I reboot
my machine (I get error messages on the line of  "Log on failure: Only an
unprivileged user can start the server"; Error 1063; Error 1069).

It appears that the postgres user sometimes tries to log on with a wrong
password (or no password?) so that I have to go to
Control Panel -> Administrative Tools -> Services -> PostgreSQL -> Properties
and retype the correct password.

I would like to ask...
- Why does this happen? (I have installed postgresql a few times on different
machines and this is the first time I see this behavior)
- Why is the postgres password wrong/missing sometimes but not every time I
reboot?

Many thanks for any explanation
(Hopefully others will benefit from it... I spent quite some time to find out
out to fix this "log on failure").

Dario