*My cousin has been a top-man in Seattle...with Microsoft for some years..
He 'tipped' me years ago...to turn off auto updates...
This is especially crucial when Microsoft introduce a new operating system...as they deliberately slow your old one to persuade you to upgrade..
I have never allowed an auto update...never done any updates...and never had any slowing or problems with any of my machines...unlike others who do the updates...*
Personally, I think it's an urban myth. In my experience, PCs just get slower over time as they accumulate more crap, the registry gets more humongous and so on. (Although I guess that some of that 'more crap' may be MS updates...) A fresh install - in my case, anyway - will always restore my PC to its former speed. Virtually all my machines are still on XP (SP2, in fact) and I wouldn't upgrade any of them because they're running slowly.
My experience is that I was given and old PC with XP newly installed. It was running desperately slowly (sometimes it could not keep up with my typing!) so I installed another 2GB of memory. That improved things a bit so in the end I bought another copy of XP from Amazon (It was a cut price one. Ex OEM) I installed it clean and let it take all the upgrades that there were. The speed was sort of OK but nothing special and it took ages to start up.
Having read the post that I quoted I thought that it might have been all the upgrades that had been installed and that the post could conceivably be correct.