Are you sitting confortably....then I will begin
The word is Trojan (based on the Trojan horse from history).
They are programs that are written to deliberately cause damage or problems with your computer (different programs do different things).
They can get on your computer in all sorts of ways, via the interent, downloading pirate software like music or films, as an attachement to an email, or on a CD or DVD that someone have given you.
That is what they are, now about your errors
continued....