Well to better answer your question it would help to know the name of the virus or trojan. This can usually be found out by performing a virus scan on your pc. If you have no available virus scanner you can pick one up from a local computer shop or major retailer. These will find, fix and maintain your computer's health. However if your looking for a cheaper route and know what your doing you can goto http://www.kaspersky.com/virusscanner perform a free virus scan. However this free service will not remove the problem but you have a name to go by. At this point I would recommend going to a search engine and performing a search such as (virus/trojan's name) removal. Many sites will provide a free walkthrough on how to remove the virus/trojan from your computer. NOTE: In many cases it requires going into your computer's registry and making edits. After you finish and close the Registry Editor your computer will ask you to restart. At this point you should be trojan/virus free. It is always recommended to perform virus/trojan/adware scans on a regular basis to help eliminate problems early on, before they cripple your computer or cause your information to end up in the wrong hands.