One of the biggest security risks for internet users, especially home users, is socially engineered malware designed to trick unsuspecting users into an action that triggers unwanted malware installation. The only defense is prevention but it can only be fully accomplished by proper user education.
Starting with simple free tools can be the first step. Suspicious and information fishing sites can be easily identified by browser plug-ins such as SiteAdvisor (I wrote about it here) available for IE and Firefox.
First infection may not be system killer but often is the culprit that brings spyware, viruses and rootkits.
Microsoft has admitted that this is a huge problem and often the only full recovery path is to completely rebuild the system. eWeek has great article in which Mike Danseglio, program manager in the Security Solutions group at Microsoft says:
When you are dealing with rootkits and some advanced spyware programs, the only solution is to rebuild from scratch. In some cases, there really is no way to recover without nuking the systems from orbit.
Dautovic 