You can set your laptop to allow only those guests you are happy to give permission to to gain access; set your guest permissions via the control panel; these are permissions which should never allow administrator permissions made available to guests.

If you do not wish to allow access then do not set up a guest permissions in the control panel.

Unless you do this; then, whilst you are using it, they may turn up wanting to use, in doing so it they would have access to your own set administration rules which is extremely dangerous; this is because they'll be able to gain access to your private files and perhaps interfere with them or place viruses on your laptop without you OR them even knowing it.

If you do succumb to bullies then make sure you SIGN OUT from your desktop then if there is then no guest desktop they are stuffed and cannot use the laptop, if they should say why do you not allow guests then simply say you've had to have viruses removed in the past and it cost you a deal of money.

You can download software that allows you to lock out your USB ports. Do a search...they are plenty of options.

I agree with some-yank answer, I would also disable autorun:

http://support.microsoft.com/kb/967715