First of all download Microsoft Malicious Software Removal tool and install it on your computer. Double click on its icon created on the home screen and run a complete system scan for Trojans and other types of malware. This software will only detect and display the names of the Trojans present on your computer and unlike an antivirus suite, it will not delete them automatically.
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Do some research on Trojans detected by the malware scanner on internet and find out where they are installed, or their file types. After doing your homework, restart the computer. Press the F8 key during the booting and select safe mode.
Now go to the location of the executable of the Trojans which will be present somewhere in the following directories "C:\windows, C:\windows\system, C:\windows\system32".
You will also have to manually edit the registry in order to completely remove the Trojan. Go to run and type regedit. The default windows registry editor will open up, delete all the Trojan entries present in the registry and exit the regedit. Restart the computer for the changes to take complete effect.
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