Problem with include, nothing running, gosh!

I guys, I'm trying to make php running ona a win machine (win2003 with
IIS6)... I've found many people with the same problem but not really a
solution.
After many tries I was able to make php running....  Great , I thought....
WRONG!
php info is running ok.
After istalling Mysql on server also that was running... super grat, I
thought....
Phpmyadmin..and the 1st problem. The problem is the same with every script
inuding other files:

Exemple:

PHP Warning: require_once(./admin.php): failed to open stream: No such file
or directory in C:\Inetpub\wwwroot\heels\wp\wp-admin\index.php on line 10
PHP Fatal error: require_once(): Failed opening required './admin.php'
(include_path='.;c:\php\includes') in
C:\Inetpub\wwwroot\heels\wp\wp-admin\index.php on line 10

Any idea how to solve the issue?
Seems a stupid thing.... but nothing is working without this (and Yes I
can't rewrite the include with absolute path for every file I'll install in
future).
Any idea to what write and where to make it running?
I'm looking on it since 3 days without results.

Mauro