A long ago we shared a solution for "Windows 7 Lock-up While Using Windows Photo Viewer" which involved dllhost.exe's incompatibility with Kaspersky Antivirus. I recently faced a similar issue where Microsoft's genuine dllhost.exe file was constantly using 25% of CPU with ever increasing RAM (a memory leak) consumption, this time it was not the Kaspersky incompatibility causing the issue but a Windows 7 bug how it handles thumbnail generation. The reason for the issue was a broken video download on my Desktop, Windows was trying to generate thumbnail for that broken/incompatible video file and failing causing the constant CPU usage - the solution was to simply delete the problematic file or move it to a inner folder in which case the problem will occur only when accessing that particular folder.