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Norton Internet Security includes the horrific Norton firewall program. It tends to cause problems getting online, and if corrupted, can result in a situation where Windows has to be reinstalled from scratch. Your best bet is to junk the Internet Security suite and get Norton Anti-Virus 2005 all by itself. You don't need the rest of the garbage that comes with the suite. If you're in cable or DSL, you should get yourself a router with a built-in firewall, which will work better than the Norton firewall without all the hassle and worry.
For the firewall, try Sygate Personal Firewall, also free.
Norton Internet Security has worked perfectly for me for a long time. Not one complaint here.