CyDoor is a spyware application which shows advertisements and snoops in on your web browser cache and history. It is not clear whether or not it actually sends that data anywhere, but it is known that it downloads advertisements even when the host application isn't running. The communications with the advertisement server can take up a lot of performance and bandwidth - if CyDoor fails to connect it may retry up to 10 times / second, causing slowdowns.
CyDoor consists of the main memory-resident application, cd_load.exe, and two or three DLL's named cd_gif.exe, cd_clint.exe and in some cases also cd_swf.exe. These handle GIF advertisements, advertisement downloads and Shockwave Flash advertisements, respectively. Downloaded ads are stored in a subdirectory to the main Windows system directory called ADCACHE.
The above-mentioned CyDoor is also installed with popular filesharing application iMesh. Removal of the spyware causes iMesh to cease functioning. In fact, even cancelling the iMesh install while it is running will install CyDoor, which will then download advertisements even though the host application, iMesh, is not even installed. Stay clear of iMesh!
If you think you may be infected with the CyDoor spyware trojan, see the cd_load.exe node for detailed removal instructions.