The answer is right there in your question:
"why do all *Windows* systems slow down over time"
It is inherent in that particular OS design. As one poster put it: "it's just a fact of life"... I'll throw in the word "Windows" in there, just to be specific. Not all OSes suffer this fate as an inevitable outcome from just using it, certainly not to the degree Windows does.
Saying that, I still do use Windows. I just don't expect it to do everything. Anything "dangerous" - like getting on the internet, reading mail, etc - I let Linux Mint handle: it's designed to deal with that safely. I actually use Windows only for software requiring hardware-acceleration, such as Poser Pro 2012, which is a Windows/Mac-only program. And since I've remove pretty much all other software, shut down unneeded "services", un-installed stuff like Photoshop CS3 and MS Office 2000 for working on VBA (which I can run in VirtualBox in Mint, my Windows 7 64-bit box-16gig RAM) generally doesn't run slowly anymore, even if it does *start* slowly. I don't dual-boot very frequently into Windows, so when I do, it has a lot of housekeeping chores to catch up on. I've found with Windows (unlike Linux), you're better off letting it do its chores first or some software may spit the dummy down the line.
Indeed, because most Windows updates seem to require a re-boot (again, unlike Linux), I tend to sort-of lose steam on whatever it was I went into Windows in the first place by the time it has finished doing all of the updates and stuff.
I guess it's a trade-off: if you're happy using the Linux answers for Windows software, you'll have a system that doesn't slow down. I find for certain software - like Photoshop - there is no Linux equivalent, but it runs okay in VirtualBox. And for the odd program like Poser Pro 2012, you can always dual-boot.
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