Q: Should I always turn off my computer or leave it on when done ?

A: Two trains of thought here. 

1) Leave it on all the time the heat will kill it.

2) Turn it off and on the Surges will kill it.

Truth is either is true, at some point your computer will die.  So the real answer is neither one matters. Computers are designed to run continually. 

However if you are going away for an extended time Turn it off AND UNPLUG IT..

Many computers come to us after a lightning storms with blown parts and we are told "we had it off".  Two cables that lead to the outside world are your electric plug and your network connection.  Make sure to unplug BOTH..

Q: I have a virus protection running and I still got infected ... HOW???

A: The best answer for this is, "wearing a bullet proof vest in a war zone does not mean you can't be shot".

Even the best Virus program can be compromised. Make sure to keep it updated. And make sure to run MALWARE or Spyware programs also. These are not viruses so your anti Virus will not always see them. The malware will get in and disable the anti virus program then let in a virus.