Port 80 only open if you are running a http server, which you use to host stuff, like my computer. if you are not running a http server on your computer, and the port 80 is open, it may be has some thing wring with your computer.
Zyxel Prestige 642ME is a ADSL modem that does not use PPOE in the firm ware.. Check your PPOE software. If not then look to see if there is a virus look for the program "Stinger" Symantec.