ME 2 Steam installation errors (Windows? Installer)

Just finished ME1 that I bought via Steam - wow! Now onto ME2 🙂

Unfortunately, when I installed ME 2, the DIrect X installed just fine, but then it tried to install something called 'Window? Installer'. This made my computer run like treacle for about 20 minutes until an error message came up saying that the Windows Installer was not accessible. The game did not therefore complete installation (though interestingly my account logged up 5 minutes of 'game time'!)

I followed this up with Microsoft online help and used their code to repair WIndows Installer in the registry....nothing - same problem when I tried again.

Using the Steam online help I verified the game cache and ran the installers for Direct X, Redistributable package and the PhysX from the SteamApps folder on my C drive....nothing - same problem. Every time I hit Play in Steam it tried to install this 'Windows? Installer' and everything falls over ☹️

I've searched the web and the forums I can find and can't see a reference to the problem anywhere else.

I'm running the following:

HP Touchsmart 320-1030

Windows 7 SP1

AMD A4-3400 APU with Radeon HD 2.70GHz

4Gb RAM

64-bit OS

IDT High Def Audio CODEC Driver 6.10.6400.0

AMD Radeon HD6410D Graphics Driver 8.892.0.0

Thanks for any help and advice