If your login id/password for your Windows computer gives you FULL access to freely change things on your computer, add applications, etc without being asked for administrator credentials, then you are VERY vulnerable to the latest generation of malware which can infect your computer. Malware is increasingly turning up on major websites rather than just in emails and infects your computer through all sorts of trickery.
Over 90% to 95% of such infections can be blocked simply by using a standard rather than an administrator level account. Sadly, by default when you first set up a Windows computer, your initial account is created with Administrator level rights. (This is not the case on Apple Mac or Linux computers, or on iPhone and Android smartphones.) Anti-virus software does not protect you against malware.
Check in your user settings and if you are an Administrator, then create a new admin account (does not matter what your call it) with the same rights and a decent password (actually, backwards version of your normal password would be okay in this instance). Logout of your normal account. Login to the new account (to check it works) and then go into the user settings panel, modify another account (namely the original) and demote it to standard/normal user status.
Logout of the new admin account and back into the usual account, check everything works. Now, whenever you try to change anything on your computer (other than just using applications, editing documents, etc) the computer will pop up a request to enter the password for the new admin account. In some cases, to install new software you have downloaded, you will need to view the file in the folder view, right click on it, and select Run as Administrator, in order to install it correctly.
Clearly, if you get an unexpected request for the admin password DO NOT DO IT.
You should normally only use your standard account for day-to-day work on your computer. Login to the admin account should be rare.