Regardless of how many safeguards you have in place such as antivirus, firewall, anti-spam, malware protection, surge protectors, backup power supply (UPS), there is always something that can go wrong. Your computer could break down, laptops can be dropped and lost. While replacing your computer is only a matter of money, replacing the information it contains may not be as easy. If you would not be willing to play Russian roulette with your computer, your best course of action is to make sure you have a recent reliable and accessible backup, especially of anything that could not or would be difficult be replace.
How long should you keep your backups? That depends on how much information you are willing to loose and how long it might take to realize that a virus has been spreading in your computer and possibly deleting your files.
Why keep old backups? Backing up a computer that has been infected with a virus will not be very useful because when you restore the files, they will still be infected.
The only way to be 100% sure your computer will never get infected by a virus is to never plug it in. Even the most popular, well known, biggest name and most reliable websites occasionally get hacked and end up distributing viruses to at least a few visitors before the webmaster becomes aware of the problem and fixes it.
Again, how much are you willing to risk loosing and think about what exactly you would lose. Do you have personal files or business files? Email? Contacts? Reference materials? Bookmarks? Bookkeeping?
Why risk your information, the time, money and inconvenience of having to deal with a virus? If it's a matter of money, there are several free antivirus programs available for Windows and OS X. Be safe. Be prepared. Use Google to search for "Free Antivirus" and the word " Windows" or "OS X" depending on your computer's operating system.
It just takes one virus to take you from loving your computer to hating your computer.