Authintication POP UP msg.

we in the help desk in one of the ISP have more than 100 call daily with the same question .. HOW MANY HOUR LEFT in my internet prepaid card???.
what i need is a script i think that can send a pop up msg to the prepaid customers once they log on to the ISP show them how many hour left in a pop up msg and the same msg will pop up agian after every one hour online.
our Databse is SQL 2000 and win2000 advanced server.

please advice me ..

Regards

Mohammed Jubara