Why and how does virus keep entering in my system that is my Computer?

I scan my PC several times in a day and atleast 6 to 12 threats are always found and i fix it. But in spite of doing that again when i check after 15-20 minutes, i find 3-4 new viruses again.. Can anybody tell me why this is happening? Cos as far as i know, virus or spyware enters when you download and save something, but not just like that. A few weeks back a very bad virus called "Zlob" %26 "VideoActive" had entered my PC. To solve the virus problem i downloaded Trend Micro Internet Security Trial version and fixed the problem of "Zlob" %26 "VideoActive". My computer was free from any problems but after a few days the virus attack has started again, in spite of me not doing anything. Somethimes i don't even browse %26 there are like atleast 6 to 12 viruses in my PC!

Technical-wise, i really don't know much about computers, softwares %26 other programs. It's all too technical for me to understand it. But i really don't understand y this virus problem is happening! pls help me!

Why and how does virus keep entering in my system that is my Computer?norton antivirus 2007

OK, your PC either has a trojan that is downloading more and more malware onto your PC, or you have infections hiding inside your system restore, or other places where they are able to reinfect.

You could also have open ports, which are allowing viruses from other infected computers to enter the open ports (think of it as an open door) thus allowing even more infections.