Installation issues - NIS compromised - lots of high risk threats coming in all the time

Hi,

I'm having a problem with Norton Internet Security. I have two computers on a wired LAN network both running Windows 7 Pro 64bit, and have NIS installed on both. For a long time (about six months) one of the computers (which I will refer to as PC1) has been showing high risk trojans and suspicoius cloud 9's on the resolved security risks section of NIS history. There are aprox. 10-15 of these showing per month. PC2, (the other machine on the network) shows only low risk resolved tracking cookies. Never one singe high risk threat in six months. I find this odd, because both computers are on the same network, but be that as it may. My problem begins when, about 3 months ago, a trojan compromised NIS and sent PC1 into an infinite windows repair loop. I ran the Norton Bootable Recovery Tool, which found and removed the trojan, but PC1 was still stuck in the infinite repair loop. I ended up reinstalling windows, which of course fixed the problem. Now, the threats had been coming pretty steadily after that, like I said, about 10 per month. About a month later, Norton won't start. I search online for a solution, and reinstall NIS. This appears to fix the problem, but soon after, the same thing happens again. I repeat the reinstallation, it fixes the problem. Then, yesterday, the same thing happened again. By this time you will guess that I am sick and tired of reinstalling NIS. So I'm here to ask: is there anything I can do about this? Why are there so many high risk threats coming in on PC1, but not on PC2?

Any input is greatly appreciated.

Thank you,

Mithrandir