I know nothing about Plus.net so I cannot comment. However, if it would give you peace of mind why not install a few of the free programs for your own protection and satisfaction. I am not sure about conflicts of programs and I know that you should not have 2 AV or 2 Firewalls on your computer working at the same time, but if your ISP is screened,at their end, that is a different matter. You actually should switch one or the other of the programs on your computer off to prevent conflicts, if you have a 3rd party FW, then turn off you Windows FW.
Freebies: Zone Alarm (firewall), ALWIL Avast Antivirus4 (AV freeware). Then there are layered protection such as Adaware, Spybot S&D, Spyware Guard, Spyware Blaster, Win Patrol,and you can go on and on.
Thunderbird is a good option, so is Firefox.