Disabling Indexing Best practice ?

Some of our users have complained about slow systems. I have disable
indexing on some to help gain better performance.

When you click OK after selecting the drive and subfolders, what I am
assuming is the indexing database of files begins deleting. There are always
so many errors deleting the files prompting for Yes or Yes To All. Do these
files no longer exist and that is why?

Is it necessary to complete this process or just Cancel the deletions.

One other question. Even if Indexing is deselected, but the service is not
disabled, is there still indexing going on?

Thanks.