Windows Vista How can I stop others from logging on?

Windows Vista here and I'm in a situation where I would like to not let others access to the internet through this computer because they surf quite recklessly and don't pay attention to the safety measures I've taught them.That's how I caught a virus this last time.
Anyway,I've tried changing the dial-up number and password and they've figured it out and logged on to the internet while I sleep.I did this because I wanted them to think(they are all minors)that I had nothing to do with it and it was a problem with the ISP or something like that...I don't want them to think that I'm stopping them from surfing because I allow it all day,just dont want them to surf at night while I sleep.
Is there anything I can do with the dial-up screen or something similar that they wont be able to figure out and I'll be able to sleep in peace at night knowing I wont wake up to a world of viruses?I'd like whatever I do to look like it wasn't me and something outside my control.
Someone help please!

 

If they have their own pass protected accounts I would have no access to know what they're doing on the internet.Then I'd really be in trouble.lol
I was thinking more along the lines of programming the machine so it would log off at certain hours and no one would be able to get on to the internet after certain hours.
I suppose there has to be a way.
Thanks Arie.

 

Well, you could just create a password for your account. Log off or shut down and they can't get in. Only way they can use it is if you start it.

 

What are the steps for this in Vista as they already know the first password I set up?
And is there any way to set the dial up screen so they cant get in?

 

I'm running Win7 so I can't say for sure but I imagine it's pretty much the same as Vista. If you have your control panel set to category view, control panel>user accounts and family safety>change your windows password. If you're using the the large or small icons view in the control panel, control panel>user accounts>change your password.

 

You could set the computer to log off/shutdown automatically at a certain time each night and only you know the password to the comptuer. This way only you would be able to log back on to comtinue using it.