Sluggish Windows 7

[Frank]

I had Win 7 machines and worked fine when purchased. Installed all the applications I wanted and still worked well. As time passed, I religiously updated windows through windows update.
Slowly but surely, the computers were slowing down. Minimised windows took ages (up to 30 seconds) to normal screen size. Clicking on the file menu would take 10 seconds. These machines would be frustrating to use. No viruses, registry cleaned with Ccleaner, no applications hogging the cpu in Task Manager.
I purchased another notebook for my wife. A low end machine with 2Gb of ram only. She just does web browsing and emailing. I asked the store people if the computer would be responsive for those tasks. Sure! Yes, the computer (Win 8.1) was responsive even after installing Firefox and Thunderbird. It was going fine. After 3 months, She noticed that it was slowing down. Sure enough, it appeared to be a different machine. It was updating automatically.
At this point, I decided that maybe the updating causes this slowing down. I turned off all windows updating an all 3 machines. I think the win 7 machines may have improved a little (I might be imagining this), but they do appear to be a little more responsive.
CPUs don't slow down, memory does not slow down, hard disks don't slow down. It can only be software.

Is there anything to this theory that windows updates actually slows the machines down?
Thanks

[Geek-9pm]

Your idea, windows updates actually slows the machines down has been repeated over and over gain hundreds of times.
Here are personal  suggestions.
Ignore most registry cleaners.
De fragment your hard drive.
Allow about  3 in. for Windows to boot.
If this is a problem, set the  PC to turn on by itself and log in automatically before you start your work time.
Schedule your updates to happen in the early morning hours.
After your PC has run for several minutes, check the response time of important programs. Bear in mind, load time is not the same thing as productivity. In fact, some very old programs load very quickly. Because they are very Spartan.*

* showing the indifference to comfort or luxury traditionally associated with ancient Sparta.

Put another way, fast loading means less luxury.

[Frank]

Just launched de-fragmentation. I started to defrag drive C: on my win 7 machine before I realised that I could just analyse first. Drive D: is fragmented 4%. I need to use it now to find out if it has improved.
Checked the Win 8.1 machine and it is setup for weekly defrag. It now shows 1% for both drive C: and D:. I don't believe that 1% defragmentation will cause that sort of slow down.
I will defrag that machine too and see what happens.

[Geek-9pm]

For what it is worth:
Optimize Windows for better performance
The above is from Microsoft and is a general coverage of the topic. Other aties say about the sane thing.

No matter how fast or shiny computers might be when they're new, they all seem to get slower over time. That state-of-the-art PC you bought last year might not feel like such a screamer after you install a dozen programs, load it with antispyware and antivirus tools, and download untold amounts of junk from the Internet. The slowdown might happen so gradually that you hardly notice it, until one day you're trying to open a program or file and wonder, "What happened to my poor PC?"

It happens. 

[soybean]

Can you be more specific regarding how you are using your computer when you encounter such severe sluggishness?  How many programs/apps do you have open at one time?  If one or more web browsers are open, how many tabs do you have open for viewing different websites? 

And, please go into more detail on the hardware specs of these computers you're talking about.

Also, what protection apps (anti-virus, malware, etc.) are you using?  Are you keeping them updated and are you running regular scans? 

The general consensus in the forum regarding use of registry cleaners is to avoid them.  Some of them - indeed, probably most of them - can definitely cause more harm than good.  CCleaner has a very good reputation but, keeping in line with the general consensus, I'll suggest you not use the registry cleaner.  What I do suggest is that you use CCleaner's Cleaner utility.  Or, alternatively, use your browser(s)' menu option to clear browser cache and use the Disk Cleanup utility in Windows 7. 

Are you using Task Manager to search for clues to what is slowing a computer?  If not, then do so.  It can be a useful tool in identifying programs that are hogging CPU and memory resources.

When Windows updates are installing, are you careful to not shutdown a computer while the updates are installing?

I"m also wondering whether your electricity supply might be a cause of computer hardware degradation.  You used the plural form of computer.  How many computers are you talking about and were all them at the same location, using the same power source?