Correctly Applying Password Expiry GPO in Exchange 2003

We need some clarification on the correct way to apply a Password policy against users in Active directory in Exchange 2003.

we are in a situation where we have an existing policy expiring password at 90days etc applied at...

<Domain>
           <sustrans OU>
                     <computers OU> *Here*

This seems to be designed to effect all AD users so their password expires at 90 days, and some complexity settings.

Why do you apply a password policy against computers, rather than users?

We have a scenario where by we need to apply a test GPO where passwords expire everyday, weve applied this test GPO to an OU where the original GPO is not inherited and placed a test pc within.

MY QUESTION IS...

Which policy expires my user password? In theory the test laptop has is going to tell me that password is expired tomorrow, but my desktop which is in the <computers OU> doesn't think the password will expire until it reaches the age limit of 90 days.