The issue is that the plonkers who build the OEM images do it in a clumsy way. I bought a Sony Vaio a few months back and the factory installed image had remains of Chinese programs and all sorts of nonsense on it.
The environments probably aren't strictly controlled and the procedures probably aren't in place.
What you then have is someone who keeps opening up a Windows image, browsing a few websites to get the likes of Adobe Flash etc up to date, running Windows update, maybe installing a few programs from a memory stick, sysprepping the image and closing it up for deployment to computers coming off the production line.
It gets worse because then you have various different hardware configurations on the PCs/laptops, so if they are lazy they will open up the same image and uninstall/reinstall drivers, expand and shrink images for different size hard disks, etc.
Oh, and bundle trial ware antivirus programs into it.
You then end up with a brand new PC/laptop that is not performing as well as it would have done if you had just installed a clean copy of Windows manually.
The sooner the Microsoft Signature
scheme comes to the UK the better - it's not rocket science, but OEMs just continue to persist bundling [censored] trialware and nonsense value-add with their systems which causes problems.
Zen 8000 Pro