In the case of anti-virus software, it's a decrease in system performance. Remember that each file you access is checked against known viruses. This takes time, and time is akin to performance. If you look at a vendor, and they don't offer this type of service, usually through the Internet, you're wasting your money. Each week, a new virus is detected, and the signature files are updated.
Viruses are a way of life on any computer platform.