you can set up multiple user accounts in windows. Just set one as an administrator account and the rest as limited accounts (assuming you're using XP). The limited users won't be able to install programs or modify system files, just browse the web, use applications and that's about it. The administrator can do anything they want. Also the cookies folders are separate for each user so they won't interfere with one eachother. Another thing that's nice about using limited accounts is that it's harder for malware to install due to the lack of priveledges.