I've long held that one of the best methods of stopping viruses and other attacks is to refrain from using accounts with administrative privileges. What most people miss about virus attacks is that while a virus does things that you don't want to have done, but it does these unwanted actions as you. The computer operating system, in other words, believes that the requests from the virus is coming from you. Just another example of how computers don't "think" like a person. The idea of "why would anyone want to do that?" does not come into play with a computer.
If you are using a computer that cannot delete/create/modify applications, it is very unlikely that a virus would be able to do so, as well. Both OS X and Windows Vista deal with this issue by running in a standard -- limited -- mode and asking for permission. Of course, if you're one of those people who automatically clicks on permission pop-up windows, this security system does little good.
Beyondtrust has just released a report that documents that removing administrative rights on a Windows computer can protect it from over 90% of common security vulnerabilities. This is higher than even I was thinking it would be but it does confirm the notion that restricting user rights is a great security strategy.