First save all important info to external drive or USB stick.
Do a start/run/msconfig-startup or services.msc to see what is automatically loading at startup.
Kill startups very very carefully.
Create a backup point first just in case.
Make sure the disk drive is ok with checkdisk
https://www.techwalla.com/articles/how-to-run-chkdsk
Make sure drivers are updated.
I got rid of Conexant because of possible keystroke logging, and installed Realtek audio driver instead.
Use free MSSE or Malwarebytes with rootkit scan or other virus software.
Windows Defender should be built in but is lightweight.
I use free Spybot S&D also.
Use
https://www.ccleaner.com/ccleaner or other and then do a defrag every night about 150 times until it can't take any more.
https://download.cnet.com/Puran-Utilities/3000-2094_4-75764049.html was lightweight and computer seemed to run zippy after.
Right click the Computer icon on desktop, chose properties, and chose Windows Experience Index to see what is part is slowest.
Press control+alt+del to get Task Manager, find the Resources button, and look at what is using all the CPU/memory/disk.
(btw I am a monkey and should not give computer advice)