If you have an iMac you don't have an antivirus unless you purchased one separately, which is not generally necessary. There is a firewall, but it is off by default. You can check by going to the Apple menu, System Preferences, Security, Firewall tab.
Error 3259 means that iTunes cannot verify your phone with its activation servers, meaning it cannot reach gs.apple.com. Check this by opening a Terminal window and typing:
ping gs.apple.com
You should get a response that looks like:
PING gs.apple.com.akadns.net (17.151.36.30): 56 data bytes
64 bytes from 17.151.36.30: icmp_seq=0 ttl=249 time=87.318 ms
64 bytes from 17.151.36.30: icmp_seq=1 ttl=249 time=87.477 ms
64 bytes from 17.151.36.30: icmp_seq=2 ttl=249 time=89.136 ms
64 bytes from 17.151.36.30: icmp_seq=3 ttl=249 time=88.925 ms
64 bytes from 17.151.36.30: icmp_seq=4 ttl=249 time=86.796 ms
If you get a timeout message this is your problem. To fix it look in the file /etc/hosts. If there are any lines that contain gs.apple.com delete them.