My PC now refuses to install anything.

I get error messages like:

C:\Program Files\AliveMedia\MP3 WAV Converter\ContextMenuHandle.dll

Unable to register the DLL/OCX: DllRegisterServer failed; code 0x80004005

Is this a registry problem?

Do I need to replace something?

Seems one of your services has been corrupted. Try a google for the error message (just the "unable...") and see if you hit oil. Service corruption normally comes from dodgy software installation or a crash.

Of course you also wouldn't be able to install anything if you didn't have admin rights, but that says access denied rather than failed and an opcode.