If you are new to PC's then the best way to learn is by your own errors and mistakes. If you have had PC's before then you will already know about much of this. But the question you ask will produce many different replies, and so it is up to you to pick through and select what best suits you and your computer.
I would say this. Keeping your computer running smoothly is your responsibility. Don't expect software to do everything for you. The most important things to know are;
1] Keep the hackers out and also protect your system from viruses and spyware.
2] Stay away from P2P download utilities like Limewire, bitTorrent, etc. You may think you want those pirated movies/music/software for free, but you do not want the viruses and spyware that so often comes bundled with the downloaded files.
3] Stay away from Internet Explorer and Outlook or Outlook Express. Use another browser type and other email software. IE and Outlook or OE are the vehicles of choice for many virus writers.
4] Backup often. You can't afford to lose those important personal files.
5] Be sure you understand what you are installing. Too often that great sounding software you have downloaded comes bundled with add-ons and other software that you really don't want.
6] Keep Windows up to date. You will need to use Internet Explorer for this some time. That should be the only time you need to use it.
7] Never open attachments in emails unless you save the attachment first and scan it with your anti-virus software before opening it. Don't trust any one who sends email attachments, not even relatives or friends, (my own experience is that their computers are the worst offenders for harboring viruses, without them knowing it).
Do not give out your email address in any forums like these, and learn how to delete emails from senders you do not know, without reading the emails first. Viruses can be installed even by previewing emails, and if you preview a spam email that in itself can tell the sender your email address is active and available for more spam.
9] Keep the dust out of your computer. Once a year open the case and remove the dust build up from fans and air inlet/outlet areas.
This post I did some time ago gives more information, but it is old and so you would need to check the list of anti-malware utilities still available. The Virus and security forum will help there;
http://forums.cnet.com/5208-10149_102-0.html?forumID=7&threadID=95901&messageID=1086971&tag=nl.e497
Mark
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