A Virus Seeks to Spread
One way to distinguish a virus from spyware is by its behavior. A virus seeks to infect a computer; to replicate; and ultimately, to infect as many computers as possible, as quickly as possible.
When you accidentally install a virus onto your computer, the malicious code that is “the virus” tries to find ways to use your computer to infect other computers. For example, an email-delivered virus (a worm) may search your computer’s file system for your Outlook address book, and send infected email messages to contacts it finds in the address book. Before you dismiss your own address book as a modest success, consider what a jackpot email addresses like email@example.com or firstname.lastname@example.org are for a virus.
Increasingly, a virus will not rely on email alone for propagation, but will try many attack vectors. These blended threats employ file sharing, telnet, FTP, IMs, or any services and programs on your computer that communicate with other computers.
Spyware seeks to embed
Viruses seek to spread, but spyware tries to stay put, a behavior we typically associate with a parasites. Parasites need a host to feed upon: nematodes notwithstanding, parasites don’t need or try to propagate. In the world of espionage, spyware is closest to a mole. A mole will avoid any activity that might blow his cover; similarly, a spyware applications is often content to hide on your system. Spyware disguises itself as a legitimate application or secretly resides as one more data link library or registry setting Joe Average User knows nothing about, so that it can collect information about you, your messaging and browsing behavior, your online preferences.
Spyware may have a heavier “footprint” on your computer than a virus: spyware will embed itself deeply into critical components of your operating system and bloat your memory with its monitoring and collection processing executables. So where virus activities are overt and sufficiently extensive in their impact to attract attention quickly, spyware activities are typically covert and their infestations are often long lasting.