A lot of people are still getting viruses and problems with ad-ware and spyware all due to lack of basic information. I am going to give you all the basics you need, along with links to free programs (yes completely free) to help you keep your computer safe.
Firstly we shall go with anti-Virus programs. A lot of you have Norton or Mcafee. Personally I do not like them, and they miss quite a few viruses. But if you're happy with it then that's alright. However there are alternatives out there.
Avast! is a free anti-virus that is constantly scanning your computer. It also has a boot scan option, ie it will scan your computer before windows even starts up. Also it always tries to update everytime your computer turns on. If you are on always on broadband this is perfect. If you are on dial up you can also manually download the updates as they are quite small and should take no more then 30 seconds on a dial up. This program is highly recommended by computer experts the world over. It is also the one I use. You are required to register it, but this is also free and gives you a code to put into your program. Clicking the Avast! word there will start a download of Avast, clicking register brings you to the registration page.
Another good alternative is AVG Free. It is not an active scanner, so it isn't always scanning everywhere. If you have a slower or older computer this is a good choice. You must manually download the updates however, and they can be a bit large for dial up, taking up to 10 minutes. However it is still a good anti-virus. This has also won praise and awards over there years, and I would recommend it to anyone with an old computer.
For spyware and adware, look no further then Spybot Search and Destroy. A small spyware program that is simply one of the best. You can set it to update manually if you wish, or everytime you run it. I most heartily recommend this. It will find things you didn't even know you have. It's layout is simple, and the tutorial a great way to learn.
If you don't have a firewall, and are on broadband, you need one. Hell even dial up should have one. If you have windows XP service pack 2, the on board firewall is basically enough, make sure it's enabled. If you want something better however, Kerio personal Firewall is the way to go. An easy to use interface, quick installation. It's all good with Kerio.
You use Internet Explorer? Stop. Right now. Go Firefox. Free, faster, pop up blockers, and not as likely to get your computer infected with spyware and adware.
Lastly a warning. Don't randomly click links in emails, or from friends in AIM and MSN, especially if the address looks any way "strange". It's a virus more then likely. Unless your friend clearly states it is something that you asked to see, ignore it.