How to password-protect your PC to prevent installation of software without your permission
Many punters are married or have girlfriends. In a supposedly trusting relationship partners share everything as they have nothing to hide from each other including their activity on a computer or laptop. Password protecting access to Windows altogether is not an option as it will be very suspicious. You could make a separate Windows account for your partner (a standard user account for example) but this once again will raise suspicions if you choose to password protect your own account. It would be like “what are you hiding in your account, that you don't want me to see?”.
Now the problem with your partner being able to have full access to your account is that she can install tracking software such as keyloggers to spy your activity. Such programs are not necessarily illegal. If legitimate an antivirus or other malware detection software won't detect them. However being legitimate programs means that they have to be installed on your PC in the standard legitimate way.
So the solution here is to prevent installation of software without your permission on your (presumably only) Windows account (the one with Administrator rights) while your partner has otherwise full access to this account. There are various ways to do this but here I explain one that is simple, effective, free and transparent.
STEP A: Disable the Windows log on password required at the startup
Question: Does your computer need a Windows password at the startup in order to log-on into Windows? If NO then move to step B. If, yes, i.e. you are prompt for a log-on Windows password this is your Windows Administrator password. Here we are assuming your wife knows this password as otherwise she wouldn't be able to access Windows (unless there are two accounts on your computer which we assume is not the case as in that case she won't be able to install anything). Now here is the trick: Disable it! Don’t worry for protection from thieves, etc. At STEP D I will explain you how to set up a BIOS password which is better anyway. The idea for doing this is that without a windows password your wife won't need a password at all to ENTER windows (she will need the BIOS one but that is independent). So she won't need to know it or in this case be aware that it has been changed (unless she tries to install something secretly).
To disable the Windows logon password (i.e. requiring password at the startup) do the following:
(i) Click Start, type netplwiz, and press ENTER.
(ii) Then uncheck (i.e. untick) “Users must enter a user name and password”.
(iii) When you click OK, you'll be asked to enter your password twice. Do so.
Here is a link explaining exactly the same thing with pictures.
Now your partner won't need the Administrative password to access Windows. So we proceed to step B in order to change the password.
STEP B: Set up or change the Administrator password
(i)Press Ctrl+Alt+Delete on your keyboard. This page will appear.
(ii)Choose “Change Password”. This page will appear.
(iii)Enter your old and new password and then confirm it.
STEP C: Prevent anyone from installing programs on your computer without Administrator password
(i) Click Start, type secpol.msc, and press ENTER. (Alternatively go to Control Panel-->System and Security-->Administrative tools-->Local Security Policy)
(ii)Then double click Local Policies-->Security Options. Here is how it looks like. Then find and double click “Choose User Account Control: Behavior of the elevation prompt for administrators in Admin Approval Mode”.
(iii) From the list choose “Prompt for credentials” . Then click “Apply” and then “OK”.
STEP D: Set up a BIOS start up password
This is a video tutorial. For BIOS password use the one you used to have as Administrator account to log on to Windows.
In the above video to get to the BIOS page you need to press repeatedly “Delete”. In your own computer it may be one of the F buttons (e.g. F2 or F10) - it depends on the motherboard. Here is list of the BIOS keys for several popular motherboards. To find the model of your motherboard do this:
(i)Click Start, type dxdiag, and press ENTER.
(ii)Then look at the “System manufacturer” (motherboard brand, e.g. ASUS, Gigabyte, MSI, etc) and “system model” (motherboard model). If you built your own computer perhaps it won't show up (it will say “System manufacturer” and “System Model” respectively. If it is a laptop it will show up certainly.