Originally posted here by SirDice
Well this is usually the case. Your machine has probably been turned into a warez monkey, a DDoS zombie and/or a spam relay.

Either case to be totally sure s/he didn't leave any backdoors you will have to backup your data and reinstall from original media.

This couldnt be more true. Other than the headache of having your bandwidth and cpu gobbled up, this could quite easily get you blocked from a great deal of networks. This becomes a real headache if you have a static ip. At my NOC office I personally block around 2-3000 spam relay bots a day. They call in here asking why they cant hit our inmail, we have to tell them to get secure and call us back.