Windows doesn't slow down unless there's something going on. As much as I am annoyed by Windows, it doesn't rot like fruit.
Windows slows down because:
other programs are running in the background
memory is low
resources are running low
the user is exposed to faster systems over time so now they can "feel" their system is too slow.
malware running on the system
the system is being hit by virtual DOS attacks, slowing the network access
the hardware is failing, causing timer interrupts, sporadic resets, or general performance issues
drivers aren't updated to address issues (video drivers, network drivers, etc.)
Is the network properly configured? If your switches are flooded and having trouble that can hurt the machine's network performance.
Also adding antivirus and other protection programs adds a layer of slowness. I mean, that's kind of a given, that if you're using a computer with a program that pre-scans all disk access to see if it's nefarious in purpose it's going to slow down performance a bit.
you can run tools like those from Sysinternals to check certain aspects of performance...like what's hammering the registry, what's accessing the drive too much, etc. (procmon, process explorer, filemon, regmon...)
Fragmented disks can cause some slowdown but it usually has to be REALLY bad to have the user use words like WOW! to describe the difference in performance.
It's popular to blame slowdowns on crap left in the registry or system folder, but Windows doesn't randomly read those bits any more than you having leftover bills or old papers around the house slows you down. More often these random bits of crap take up drive space, contribute to fragmentation, and create stability issues than anything else.
It's a good idea to wipe and reinstall just to get rid of drive bloat, fragmentation, and leftover crap the OS doesn't need more than speeding up the computer unless you've got programs installed eating the processor, memory, and there's malware. Just using the same system as a typewriter for a year and basic web browsing/email (as long as it isn't infected with something) shouldn't affect the speed. More likely the user is just perceiving a slowdown.
If you're really curious you should probably benchmark a system then do a wipe/reinstall and re-benchmark it to see if there is a difference.