|Registry cleaners can find all sort of Registry entries it considers useless, and references to files that are missing Those are almost all placed in the Registry by poorly written software installations, or left behind by poorly written Un-install programs. I just delete the entries for what they find are missing and I've never had any problems because I did that. |
If you're not getting any messages about missing files in Windows itself, ignore that info, or delete the Registry entries for it.
There are zillions of things that can slow down Windows.
A common one with laptops, or desktop brand name systems that have only one hard drive, is you don't have enough free space left on the Windows partition.
How much free space do you have on C, and what's the total size of C ?