This is likely because somehow your computer has been set to open up screensavers with notepad . Programs like system mechanic do this to prevent malicious ( definitely not this one ) screensavers from exploiting security flaws in windows . For now you can work around it by renaming the program replacing .scr with .exe , or if you are a more advanced user , you can go here -
http://www.dougknox.com/xp/file_assoc.htm , and download the SCR file association fix . Unzip it , and import the file to your registry . As always the registry should be backed up first .