I was trying to add a new COM DLL to my VS2010 project but it kept giving me the following error message:

A reference to the “….dll” could not be added.Please make sure that the file is accessible and that it is a valid assembly or COM component

Then I realised I hadn’t registered my self-registering assembly on my machine as this was a new machine and the first time I had used this project. Since I had my VS Command Prompt open I ran RegSvr32 on the assembly and waited for the usual message. Instead it threw me an error and I was left to try and find out what was causing the problem. As it turns out the solution was relatively simple. I needed to run my command prompt as the administrator to get RegSvr32 to work as I am using a Windows 7 OS. Now my COM DLL is registered and listed in the COM tab of the Visual Studio Add Reference dialog.