Often, screensavers can be harmful files. However, you should take GOOD care before installing a screensaver.
Windows 7 makes installing screensavers as easy as 1,2,3. Advanced users will get instructions how to install a logon/logoff screensaver. Intermediate users might want to read about customizing their screensavers (e.g. Here’s a short how-to guide that will give you step-by-step instructions.
Installing screensavers on Windows 7 is really simple.