If you're running antivirus software from two or three years ago, you should upgrade to the most recent version, even if you still receive up-to-date malware signature files for the older edition. The underlying technology for antivirus software has improved significantly in recent years.
To detect threats, antivirus products today don't rely solely on the traditional signature files (regularly updated files that identify the latest malware). They also use heuristic techniques to detect and block infections that no one has seen yet. Given how frequently new viruses crop up in the wild, the ability to protect against unknown malware is critical.