Step 1. Open up notepad
Step 2. Type

lol=msgbox("Whatever you want message to say",20,"Whatever you want the window to be titled")

Ex:
lol=msgbox("Warning your computer has been infected by a virus ",20,"Virus Alert")

Step 3. Save the text as Whateveryouwant.vbs


The number 20 represents the type of window style the message will be in. You can experiment with different styles you want.