Well you do mention that you visit some high risk areas prone to infection. Either you have one of the best products installed or one of the worst and are not aware of malware infestation present in your computer system and files. Your pc may have even been hijacked into a botnet (zombie network) which would explain never detecting threats. If by "torrents" you mean yo are in a Peer To Peer (P2P) netowrk - then you may not be aware that they have full access to your computer. You will notice by right clicking Properties in that area that it says "This Program is connected to another Computer with access to your computer" .... or along those lines. This is the only way to join a P2P.
By your own admission of not practicing "Safe Practices" - or you may think you do - I would really doubt that that pc is not entirely infested with malware and is actually now part of a botnet.
TRY
Online Tool Developed to Check for Botnet Activity [wrkx w/ Netbooks]
BotnetChecker.Com
Go To: http://botnetchecker.com/
PRWeb via Yahoo! News Wed, 12 Dec 2007 5:00 AM PST
http://news.yahoo.com/s/prweb/20071212/bs_prweb/prweb575432_1
It is estimated that 1 in 4 computers on the internet today are part of a botnet. After observing bot activity from thousands of compromised computers, local
administrator develops easy way to check for botnet activity.
Trend Micro RUBotted (free) 4-5* (Detect only) [wrkx w/ Netbooks]
http://www.trendsecure.com/portal/en-US/tools/security_tools/rubotted
Malicious software called Bots can secretly take control of computers and make them participate in networks called “Botnets.” These networks can harness massive computing power and Internet bandwidth to relay spam, attack web servers, infect more computers, and perform other illicit activities.
Security experts believe that millions of computers have already joined Botnets without the knowledge of their owners. By using remotely-controlled computers, the criminals in charge of the Botnets try to remain anonymous and elude authorities seeking to prosecute them.
RUBotted monitors your computer for suspicious activities and regularly checks with an online service to identify behavior associated with Bots. Upon discovering a potential infection, RUBotted prompts you to scan and clean your computer.
P2P file sharing: Know the risks - Microsoft Corporation
http://www.microsoft.com/canada/protect/protect-yourself/protect-your-data/article.aspx?article=p2p-file-sharing-know-the-risks
Zombie computer
From Wikipedia, the free encyclopedia
http://en.wikipedia.org/wiki/Zombie_computer
Zombie network
From Wikipedia, the free encyclopedia SEE Botnet
http://en.wikipedia.org/wiki/Botnet