1) my pc is connected to other PCs on the network.
OK. Look at the advice you were given here:
http://forum.avast.com/index.php?topic=37217.msg311701#msg311701 Is my computer infected with virus?
Ans: Not necessarily. It is more likely to be due to an infected computer in your network.
2) Is my ISP inserting the javascript line?
Ans: Definitely NO. This problem is caused by computers infected in your physical network. Your ISP does not insert js into your pages. It is illegal to modify data in any way by the ISP, except for blocking certain data which is likely to cause problems.
Another computer on your network is injecting the malicious script into web pages you visit: you must disinfect all PC's on the network.
