Got this definition straight off the web.
pronounced as separate letters) Short for original equipment manufacturer, which is a misleading term for a company that has a special relationship with computer producers. OEMs buy computers in bulk and customize them for a particular application. They then sell the customized computer under their own name. The term is really a misnomer because OEMs are not the original manufacturers -- they are the customizers.
I am sure if you wanted to go back to Windows XP home/pro SP1a, you
couldn't do worse than give E-Bay a try.
Cheers from another Aussie