The first thing that you need to know is that you cannot protect against every possible exigency.
Determine your tolerance for loss
First determine how much you can afford to lose - be it an hour, a day, or a week, then concentrate on protecting the rest. You should determine the cost of a loss – both physical loss and data loss, then you should budget less than that for protection.
Should You Back up your data?
Back up any of your data that you cannot afford to lose. Depending on your operation, you may want to have your data copied to several disks immediately – it's called a RAID – or daily or weekly backups may suite you fine. I've come to the conclusion, that off-site backups are a necessity. I have seen too many clients back up their data religiously and then have a thief steal both the computers and the backup device! Off-site storage is comparatively cheap now days.
Make sure to obtain all of the free patches and upgrades to which you are entitled - especially if it is a Microsoft product. Make a practice of doing these upgrades and patches every week.
Do install a reliable antivirus program. Good ones, both paid and free are abundant. Make sure that you update it regularly – at least weekly! Choose the automatic update option for the maximum protection. Run a complete virus scan – overnight is my choice – every week at least.
Email is the most common way that viruses get spread. Your antivirus program will be a great help here as it will check every incoming email for viruses. Never open an attachment you are not expecting, even if it is from someone you know.
A good SPAM filter can be useful – maybe. I find that a SPAM filter is good if your email always comes from people in your address book. If your email is like mine, and comes from different people all the time, SPAM filters are not that useful.
NEVER put your good email address on the web! Spammers will harvest it almost as soon as it is up, then the barrage of SPAM will really start.
A little trick that I have used to cut down SPAM is to set up a separate, throw away account that will be seen by web bots. Change it regularly.
Now, I prefer to use a support ticket system. Neither users nor bots see my email address – they just fill in a request that can either be emailed to me, or held on a server for me to fetch. I use a script that cost me $7 HERE.
Almost all of us are subject to frequent minor power failures as well as the occasional major one. We frequently see surges and spikes, and although a good surge protector can clean out most of the spikes, it is useless if the voltage drops. A better idea is an Uninterruptable Power Supply (UPS) with Automatic Voltage Control (AVC). These normally change the AC mains voltage into clean 12 V DC then turn it back into clean and well regulated voltage for your computer. A good, single computer UPC with AVC can be found for under $100 here in Canada.
Wired networks are usually fairly secure, because the router generally provides address translation which makes it difficult for hackers to reach your computer. If you are using a single computer, you should seriously consider installing a router between you and the Internet.
Wireless networks can be a nightmare. You can drive up and down any street in North America and find an open wireless network. Just what the hacker ordered! Your wireless router comes with security encryption. Use it.
If you follow this advice, you should have fairly secure computing.