REGSVR32: the module "xxxxx.dll" failed to load ... dependent assembly could not be found

This is almost certainly due to a missing dependency. Use a tool like Dependency Walker to find the required dependencies of the DLL. Or, if the DLL is supplied by a third party, read their documentation which should specify the required dependencies.

Note that Microsoft.VC90.ATL indicates version 9 of MSVC which is VS2008. So you would need to install the MSVC 2008 runtime to meet that dependency. It looks like you installed the MSVC 2010 runtime by mistake.

Finally, please don't put files into the system directory. It belongs to the system and should not be modified by you.

Dependent Assembly Microsoft.VC90.ATL,processorArchitecture="x86"

This indicates missing Visual C++ runtime module (see Visual C++ Libraries as Shared Side-by-Side Assemblies). It is available as redistributable installer (this is presumably the one you need; x86 variant), which you need to install before registering your DLL (which is in turn dependent on missing component).

There is also another reason why this fails. I just ran into this myself. I was using API methods that did not support Windows 7, such as PathCchRemoveFileSpec which I had to update to the older, deprecated PathRemoveFileSpec. I used Dependency Walker to verify this was the cause. Dependency Walker lies a bit. If you look at my screenshot (below), the red section shows the actual problem - the methods it wasn't able to resolve in DLLs it has, etc. - but the blue shows DLLs that are actually not the problem at all (these DLLs exist). Since Dependency Walker is so old, it lies a bit (its outdated, it thinks something isn't available from time to time), but usually, it also tells the truth...you just have to scroll down a bit from the top left pane like I did. Note to self: please scroll down next time. Anyways, once I ditched pathcch.h, everything worked on 7. Happy coding.