Do Not Be an Administrator

You should not be logged in as an administrator on your computer. While you are logged in as an Administrator you have permission to initiate any task that the computer is capable of performing. You have permission to almost all of the files on the computer and the registry. What is the big deal? This is how a computer gets a virus or gets broken. You can install programs on your computer by simply visiting a website. Windows Vista and 7 both have UAC (User Account Control) which reduces the risk of installing programs without your knowing.
To reduce the risk of unwanted software being installed on your Windows XP, Vista or 7 computer make yourself a regular user and set the Administrator password to something that you can remember. When you need to install a program Windows will ask you for the Administrator password then it will install. Windows XP is more complex in handling administrative permissions which is why users are often administrators over their computer. When a program needs to be installed with Windows XP you often have to give yourself administrative permission again and set the permissions over folders and registry keys then return to being a regular user.
While doing these steps adds additional work to install software that you do want, it prevents installing software that you don’t want.
P.S. While running as an administrator Windows XP is the least secure of the three operating systems.