One way that windows 7 (in particular) slows down, comes from the use of the winSXS folder.
Basically, because the windows software ecosystem is so... Plagued.. with legacy software that expect older versions of system libraries, Microsoft invented a solution to detect those dependencies and satisfy them with those older libaries in a sandbox-- the WinSXS folder.
As time passes, and updates happen, system libraries get updated-- instead of being replaced, they get moved to the winsxs folder and archived. This is so when your bitchy internal-only legacy application that is oh-so-mission-critical that it simply cant be rewritten for a modern OS gets run, it can continue to run.
The downside is that as this treasure trove of old libraries grows, the penalty of the checking routine becomes more and more apparent. (also, it consumes more and more disk space.)
Other forms of slowdown are not specific to windows 7 and newer however.
The registry is a binary file that must be parsed to find entries inside it, and it too can become fragmented. As changes are CONSTANTLY happening to the registry, the (actual) structure of the registry can become more and more byzantine. Since such changes are completely unavoidable with daily use, the slow degradation of this system is also unavoidable unless you boot from a golden image each and every time. This has been a problem since at least the 9x days. Back then, you could automate registry defragmentation with a bootup script because of the complete lack of filesystem security on FAT-- (Tell regedit to dump the registry in its totality into an exported text file, then tell it to rebuild the registry from scratch using that text file dump, then cleanup the temporary files afterwards.) You cant do that with modern flavors of windows because 1) you cant invoke scripts that easily on bootup anymore 2) the registry files are protected with NTFS security descriptors, 3) the OS locks the registry basically as soon as NTLDR finishes, so you cant replace the registry files while live.
There are of course, the other causes of slowdown that come from cumulative misconfigurations that happen from automated updates, but meh.