2 Viruses on my pc in 2 weeks

I have the latest edition of NS. Most of my settings are "out-of-the-box". I made a few settings stronger.

In the last 2 weeks I have gotten trojan.gen.npe and trojan.gen.2. NS did not block these and only picked them up after doing a full system scan.

Why didn't NS block these? Do I need more protection than NS? What?

Been using pc's since win98 first came out and never had a virus until now.