First, make sure you have a firewall, antivirus, anti-spy, and anti-ad software, and keep them up to date.

Create restore points on your computer weekly, so that you can restore your programs if need be.

Refrain from downloading free music, games, and offers from unknown, unsecure sites.  With them often arrive Trojans, viruses, and pop-ups.

Make sure your computer equipment and software is sufficiently robust for the amount of work it processes, to prevent crashes and loss of information.  There is no better investment than proper equipment.  If you can't start with what you need, plan to upgrade at the earliest possible time.

Back up your documents, contacts, and all information regularly—at least monthly, but better weekly.  For this, I burn separate rewritable DVDs (CDs work, too) for each of my business areas. 

Also back up your software programs and keep all of your software CD’s under lock and key, labeled, and preferably recorded in a database such as Access or FileMaker Pro.  Record all key codes with each CD and in the database.  

It is also advisable to burn a second set of data and software CD’s and lock in a separate location, in case of  fire or other mis-adventure.