Last year, my it teacher told me that we can kill the process explorer.exe, and if another process starts using more cpu/memory it means that its a malware or virus.
Does this actually work ?
Not really. There are many reasons why a process may be started without your input, so to speak. In Unix/Linux terms, these may be daemons - processes that run on a regular basis in order to do system cleanup/maintenance duties, or to provide other services. The explorer.exe process for Windows is your user interface. In fact, if you shut it down, it will probably be restarted, and (initially at least) may require more CPU and RAM than it did before, simply in order to get back to a "steady state".
So, may I ask where this question is coming from? Do you think you have a virus? If so, what have you done to determine what it is, and what has been infected (beyond the system in general)?