Actually it was third party code, mostly custom programs written for Fortune 500 companies, that amazingly is still in use today despite needing kluge after kluge to keep working. That software is why Microsoft still ships 32-bit versions of Windows, because Win64 can't thunk down to Win16.
If you need to disable the constant complaints to upgrade to Windows 10, just install the GWX control panel:http://ultimateoutsider.com/downloads/
You may have to run it periodically to re-enable the blocks since Microsoft is getting pissy about nobody upgrading to 10. Its gotten so bad they're resorting to malware tactics to get people to install it. As a result of their boorish (and borderline illegal) behavior they periodically flip WGX back on, because, y'know, people are "accidentally" disabling it.
Personally while I managed to hack in Windows 10 onto my MacBook, my actual PC will bluescreen during 10's installation. As a result if I let it get into its endless upgrade cycle I end up wasting hours waiting for it to upgrade me to 10, bluescreening during install, then rolling back my windows 7 installation... only to start all over again a few days later.