Practice Safe Computing.
Always use anti-virus software and update the file it uses to recognize viruses; use a firewall and keep it updated; and, keep Windows updated.
Keep all of your software updated, especially any programs that access the Internet (Microsoft Windows, Microsoft Office, web browser, email program, anti-virus, firewall, instant messengers, Adobe Reader, etc.).
It's only a matter of time before you get a virus, worm, adware, or some other malware program. Be prepared.
Keep multiple backups of critical data. (I believe in healthy paranoia when it comes to computers!)
You don't have to backup your programs, only your data files and system files (in other words, files that change or that you create).