think of a trojan horse like a family tree where there's 1 there's 2 then 4 then ... then you need a new hard drive because its done been fried. leaves more access to worms, bombs and a bunch of stuff that you don't want. my friend did that once. He left a trojan and then his hard drive crashed and we got it up and running scanned it and found over 140 trojans 10 worms and right after I saw him throw that hard drive into the trash.