Cydoor is an ad-server put on your system by programs like iMesh and KaZaA. It's a nasty little .DLL that screws with a bunch of things.
How to remove it:
1.Delete the following files (usually found in C:\WINDOWS\SYSTEM\):
2.Delete the ADCACHE folder and its contents (usually found under C:\WINDOWS\SYSTEM\).
3.Remove Cydoor and Cydoor Services from the Windows Registry. Usual location in registry: