What is the modus operandi for viruses?

What is the modus operandi for viruses? As a previous software technician for Dell on Call, I cleaned Dell customer’s pc from viruses manually and thru a bunch of tools, so Id like to share the following excerpt of reasons why people make viruses and the different types of virus writers taken from the website:

The 8 reasons Flock is my Favorite Browser of choice

1. based on Firefox 3.0 (need I say more) 2. incorporates yahoo mail and gmail into its system (my favorite email providers – sorry hotmail a este live mail or msn mail but Gmail is the best email system and i love using yahoo messenger and text messaging inside yahoo mail. 3. It has facebook