Stop people from messing w/ my computer!

There are other people in my house that try to use my computer. I don't want my younger sister using it, and I always leave it on. So I figure the best way is to have a screensaver PW. However, I'm running Win XP and can't get this to work. I can choose "PW protect on resume in screensaver options, but it doesn't do anything. and it doesn't have a "Set PW" button. Why did they do this? Does that mean I have to have a logon PW and everything too? That is really stupid. Any help appreciated.

make a logon password...when you want to leave your computer, hit Winkey + L to lock your computer. If your younger sister has an account on your computer she can still log in but it won't disturb your stuff. If she doesn't have an account she is SOL :D