Does anyone know where I can get a copy of the Advent OEM vista Home Premium disc?

I have a problem with Windows Installer and have been in contact with Microsoft for around 6 weeks. Finally they have decided that they can fix the problem with the original Vista software but my machine came with Vista pre-loaded.

The Tech Guys say that I need to do a clean install from the back up created when I bought the machine but this would reformat my hard drive and loose all the software and settings I currently have. (the 60 GB of data is backed up on an external hard drive).

Any ideas?

Thanks in advance.