Technically you can install Norton internet security on two PCs, Legally it depends on the licence.
Most software for home use is limited to use on one PC only, so check the box your software came in, it will say in the licence agreement how many PCs you can use it on.
I will pre-empt many other users on this forum and suggest that you dump Norton, which I, along with others here, think is as much use as a chocolate fireguard, and instead install Avast! for anti-virus and Zone Alarm or Sygate Personal Firewall. All thses products are free and can be installed on several PCs at once.