You dont say if you have XP or Vista.
Here are instructions for creating a new user under windows XP, and Vista is similar.
http://www.microsoft.com/windowsxp/using/setup /winxp/accounts.mspx
Make sure you set a password on your main account to stop anyone else logging on.
Here in my house I have 5 userids set up on Windows, one for each member of the family.
When they want to use Windows they can logon as themselves.
Great thing about this is that each of them can have their own email setup so they can only logon and see their own emails.
Also they can have their own browser favorites.
Also they can have their own dekstop picture and so on.
They can have their own totally unique Windows environment, and anything they do does not affect anyone elses userid.