I explain the basics of it in this wildly unpopular thread:
here
Most people are blissfully unaware of what can be done to their systems. I always run WinTop to detect any program or service that is functioning. I know them all, and if even one little screwball trojan is activated, i know it right away. I do not run any computer, government, corporate, or private without monitoring all porgrams that are activated and run.
Most Trojans come from one of 3 sources:
1. In a downloaded program, like a No-CD ROM patch. A patch for Quake III is the only time any trojan (BackOrifice, as I recall) managed to make it into my system. I immediately cut it out of the registry, tracked down the offending file, and have it in special storage for reference (technical) purposes.
BTW, AtGuard is also a firewall, if you choose. It can block BackOrifice, NetBus, and all the ugly variations of them.
2. By running an e-mail attachment, or in some cases, even opening an e-mail.
3. By exploitation of weak operating systems, e-mailers, and browsers. The king of this category is, naturally, Microsoft. MS even distributes virus-infected software on some of it's CD ROMs once or twice a year, but usually to overseas customers, partiularly in Europe and Asia for some reason. Of course, they to not attempt to do it on purpose, but nevertheless, they do infect some computers. OSs like Linux, Unix, and FreeBSD are, by the skillful nature of the OS design, almost invulnerable to all exploits and viruses.... esp. compared to the numbers that florish with the help of MS's weak Win9x & NTx systems & broswers. To defeat hackers (assuming you runs MS Applications or OSs), you need the assistance of programs that are competant in such matters, like AtGuard, BlackIce Defender (a firewall, not really an ad blocker), ZoneAlarm, etc.
Without the help of programs, you must learn to edit your registry.... which is safe to do if you make backups (and know how to restore them), and are careful in what you do in the registry.