Are there any compatibility issues/conflicts with the Norton Internet Security firewall?
Shouldn't be but can't confirm. I run them with McAfee Antivirus and ZoneAlarm's free firewall without any problem.
Weren't for me. But you do want to watch out with Hijackthis, which is a regeditor and could disable programs you need if you aren't careful. Many sites on the web have folks willing to help you figure out what you have in your startup files. I don't know exactly all the stuff myself, but I have found a lot of friendly people willing to assist me when I catch something weird with that--and I highly recommend if you use it and aren't 100% computer literate, you don't go knocking off stuff without a second opinion first.