We all run into problems with our computers from time to time. You needn't fret however, as there are tools available to fix your machine. If you want a guideline on the general process of troubleshooting your machine, then go here.
For the most part, computing problems fall under a small handful of catagories. With a little knowledge and the right software, we should have you up and running in no time.
Antivirus
AVG 8.0 Antivirus (free)
Microsoft Security Essentials (free)
Avira Antivirus (free)
Eset NOD 32
Kaspersky

Comments: The best antivirus in my opinion is NOD 32 from Eset. It is very fast and easy on system resources. It is the antivirus that I use on my main machine. It is not free however, so in lieu of that I strongly recommend Microsoft Security Essentials. I use Microsoft Security Essentials on all of the other machines in my house, and I have used it for a while during the beta. I strongly recommend it, and on the upside you can get off of the yearly subscription train. As a note, a very good commercial alternative to Eset NOD 32 is Kaspersky. Pick only one and go with it.

Antispyware
Windows Defender(free)
Webroot Spysweeper
Spyware Doctor
Lavasoft Adaware Personal SE (free)

Comments: Unlike Antivirus, you can (and should) run more than one antispyware tool. Windows Defender is quite good and is included in all Vista systems. You can follow the link if you have XP. The two best antispyware programs are Spyware Doctor and Webroot Spysweeper, but keep in mind that neither of them are free. If you follow my recommendations for secure computing habits, you will be fine using Windows Defender and Adaware.

Firewall
Comodo Personal Firewall (free)

Comments: At the very least you should have Windows Firewall turned on, but if you want even more protection the firewall from Comodo is top-notch. For the average user who is connected to a router, this is likely overkill however.
Web Browser
Internet Explorer 8
Opera 10
Firefox 3.5

Comments: If you are using any browser other than the ones above, then you need to IMMEDIATELY download one (or more) of these browsers. I use them all, and my personal preference is for Opera and Internet Explorer (I use version 8). Firefox is great in its ability to support an almost limitless amount of extensions that turns the browser into a swiss army knife of web browsing, but be careful because if you add too many, you can slow down your system.