Why are there so many user accounts on my machine?

As with so many things, the answer is: it depends.

To begin with, Windows has a slightly different idea of what it means to be
a “user” than you and I do.

Add to that the various and sundry software packages that you might have
installed and the number of user accounts can get … interesting.

A user is a user is a …

You and I naturally think of a user as a person who uses the computer. In
most cases, there’s only one user and it’s you … or me.

The separate “Administrator” user account is something that I think most
people at least somewhat understand as simply a different login that might have
more privileges associated with it than our normal account.

Typically, it’s the same real-life user; we might choose to login to our
machines as the Administrator or with an account that has administrator rights,
but to Windows, it’s actually a different “user”.

So, Administrator (which is often hidden) and our own login account – we
probably understand why they’re there.

Then it gets … interesting.

Additional Windows accounts

One account that you may see from time to time is called the “Guest” account.

Ultimately, it’s an account created to allow individuals or other computers
who don’t have accounts on your machine to do things with your machine
anyway.

For example, you might make your printer or a network share available on your
local network and allow anyone who can connect to it to access it. The
Guest account is one way to do that.

More often than not, the Guest account is disabled. Even with restrictive
security settings in place, it can be more of a security hassle than it’s
worth.

Additional software accounts

Occasionally, software that’s installed on your machine needs to “look like”
a computer user. It’s one way that software can sometimes be granted privileges
greater than what your login account might have without granting the software
full, no-holds-barred administrative access.

The .NET framework seemed to be particularly prone to adding user accounts
for various reasons, although in later versions, less so (or perhaps Windows 7 has
done a better job of isolating them in a not-as-visible way).

Another somewhat common scenario is backup software. Backup software often
needs lower level access than your typical login account. As a result, when
setup is run, it creates an additional user account for the backup software.
The setup, typically run with full administrative access, creates a new user
account that (as I said before) has fewer privileges than Administrator, but
perhaps a couple of additional privileges than a limited user account might
have.

The backup software then logs in using its own account when it performs
its actions.

It’s all about security

It’s really all about security and making sure that the various software
packages have the appropriate level of access; they can do the things that they need to in order to do their job without being granted access to things that they don’t.

The concept of “user” is really a concept of “account,” and thus, accounts
are generated automatically that may never ever be used by a real human user
directly.

Booting users

You asked how to delete unnecessary user accounts and for that, I have a
simple answer:

Don’t.

The key is really understanding which ones are truly unnecessary and for
that there’s no simple answer. Sure, if there’s an XYZ User that appeared
after installing XYZ Backup, then it’s probably safe to delete that user after
uninstalling the backup program. But that’s about as good as it gets.

In most cases, it’s not at all clear which are unnecessary, mostly because in
most cases, none of them are. Yes, each might take up a little disk space (often
not as much as you might think), but even if they do, that account and that
disk space is very likely part of what’s required and used to provide a service
on your machine.

Deleting that user could easily cause something that you care about to fail.

Thus, I recommend not deleting them at all; the only caveat being to make
sure that you are keeping your computer properly secure and malware free.
Malware could, of course, create user accounts to hide or otherwise obscure
what it’s up to.