Next to creating regular backups as I explained here it is important to keep your computer virus- and trojanfree.
It is true that if you would get an infection you can use Macrium Reflect to go back in time and instantly get rid of any virus you might have caught, but if you do that without antivirus programs, you'll be endlessly replacing your system with images and never getting any work done.
Besides - even your external drive with the images might get infected if you let things roam free.
In fact, that is an important part of the reasons I don't recommend using system restore, because it is part of your systemdrive and might easily get infected / damaged by the infection, which results in the impossibility to restore.
Ok, enough scaredy talk. Let's see what we can do.
Nowadays you don't have to spend a fortune on antivirus programs, there are enough free options that keep you perfectly safe, so let's get started!
Basically there are three things you need:
1) a virusscanner
2) a firewall
3) a malwarescanner(s)
VIRUSSCANNERS:

When you have a legit version of Windows (I wouldn't expect anything less from you!) you can use MSE (short for Microsoft Security Essentials). It'what we recommend here at HTG :)
Here is a tutorial on how to install and use it.
Another good free virusscanner is Avira.
Here are two slightly older but very good tutorials on how to install and use Avira:
There are several other free virusscanners available, I just happen to like these two because they are relatively light on resources and do a good job.
Just keep this in mind:
- Use only one! Installing and using more than one will NOT give you better protection. It will only mess things up.
- the default settings ensure you regularly get the latest updates. That's very important. Wrongdoers find new ways to attack on a daily basis, so the updates should come in just as fast ;)
- sometimes a virusscanner gives a false possitive. That means it sees something as a virus, which you doubt is a virus, like a program you've been using for ages and you get a new virusupdate and all of a sudden your virusscanner tells you that program is evil. It doesn't happen often and if you think it is a false possitive you can always ask here or on the forum of the virusscanner.
- most virusses and trojans get on your computer by users! So be careful:
On this site, about halfway down under 'Human Behaviour' there is a good list about the do's and dont's you should keep in mind :)
WEB OF TRUST:
To know which sites are safe or not is a little bit easier these days. You can download the Web of Trust (WOT for short) add-on for your browser and it will show you with a little green (= safe) ring or orange (= dodgy) one or a red (=bad bad link!) what the quality of the site is you want to visit.
So it will be easy to avoid the bad ones!
FIREWALL:
Another thing you need to keep your computer safe is a firewall.
A firewall protects you from unwanted visitors from the internet. More about what a firewall is / how it works you can read here under 'Software Firewalls', but my main point is you need one :)
Fortunately in the newer windows OS Vista and Se7en there is an inbuild firewall.
Go to Control panel > Windows Firewall and make sure it is set to 'on', like this:

On the lefthand side you can click 'Turn Windows firewall on or off' to turn it on if it's not.

And you can click 'Allow a program through Windows Firewall' to add programs you want to allow internet connection or block the ones you don't want to.
Mostly Windows handles this by itself though and it does a pretty good job :)

If you don't want to / can't use the Windows Firewall there are some freeware options, but often they are a lot heavier on system resources. So if you can use the inbuild Windows Firewall.
A free alternative is ZoneAlarm Free Firewall.
MALWARESCANNERS:
Even with all these measures you might attrack a virus or worm or trojan. If it's not you, then someone else who uses the computer - friends come over and their kids play a game on the computer or download stuff you wouldn't want to touch with a ten foot pole etc. etc.
It doesn't matter how, when it happens you want to be prepared for action ;)
a) give your system a full scan with your virusscanner
b) download and install SUPERAntispyware (SAS for short), let it update and run the scan.
c) do the same with Malwarebytes. Mind you: don't run them together - first one, then the other.
d) if your system is badly infected get the portable version of SAS and put it on an USB stick and run it.
e) or if you've prepared yourself well, get out the Macrium Reflect bootCD and restore an image you made earlier, like I advised here and be back in a smoothly working environment in the shortest possible time with the least effort :)
(I tend to do the dishes when restoring an image and be up and running by the time the last forks and spoons go in the drawer. Now how's that for getting out of trouble!)
SOME MORE LINKS ON HOW TO REMOVE FAKE ANTIVIRUS AND OTHER MALWARE:
How to remove fake antivirus.
How to defeat fake antivirus.
How to remove security tool and other fake antivirus.
And here is a single link to an example of these scareware scams - it's no use giving you a full list, since they change all the time.
Just note that they contact you when you're not looking for them and most of the time they want instant money for instant help.
DO NOT GIVE THEM YOUR CREDITCARDNUMBER!!!
