Trojan Horses
One property that actually separates a Trojan Horse infection from a virus is that it is non self replicating but can still do just about the same amount of damage when it infects your PC. These infections are known security hole breakers in that they mostly give hackers the power wither access your system or gain remote access to it from another point.
This will obviously then allow those malicious users to remove various system data, steal information, change settings and so forth. Internet security software with virus removal capabilities are most often required to remove a trojan infection because many simple anti spyware program do not possess the ability to do so.
Furthermore, once access is gained more harmful files could be downloaded to your system making it suffer from further degraded speeds and usability. Watch out for frequent rebooting and changed desktop wallpapers because these are 2 symptoms which are usually indications that you could have a Trojan on your system.