Someone briefly referrenced Greek Mythology but really didn't answer Harries question.
In Greek Mythology the Greeks and the Trojans fought a long war, the story involves, sex, money, greed,
war, betrayal...sorta like The Jerry Springer TV show, if you get that in Denmark.
One side built a wall around their city, the other side wanted in and couldn't get there, so they pretended to go away, but they left a giant wooden Trojan horse behind outside the gates to the city, the people thinking the other side had gone, opened the gates and brought the large wooden horse into their city.
Once inside, the other sides military hiding inside the hollow wooden horse came out under cover of night and opened the gates to allow the large army hiding outside to come in and kill the cities people.
Thats the very short version with all the greek god and goddesses names left out.
A trojan pretending to be a free good download, it gets into your computer because you clicked on something free that looked legitimate but was not. Once inside your computer the army of bad guys (viruses) set about letting more and more bad guys in, to destroy your files, programs and drive you crazy. They (viruses) are expert at hiding in your computer.
That's why you came to this forum. Perhaps one of these very smart people will help you to run some programs to help get rid of them. How about it guys?
Good Luck Harrie.