Controlling User Accounts

Hi. Here's what I need to do:
1)Allow for multiple users to access my elderly computer.
2)Restrict usage of the network so only I can access the internet.
3)Install copies of Windows 2000 Pro on two hard drives on one computer so that I can boot from either hard drive.

Ok, here's why. My stepmom has an aging computer that she uses for her job. She only uses it to access a VPN network. Her computer would be a model of cleanliness if she were the only user. She's not. She has a lot of grandchildren that come to her house and download PORN, AT THEIR GRANDMOTHERS HOUSE. Needless to say, they are stupid and use the virus sharing program known as Kazaa. Her computer is always dirty and I have to fix it (err...reformat it) every 2 weeks. I'm busy and this usually takes me two days to get to it. I need to make her computer virus free permanently. Can I disable the network for different user accounts in Windows 2000? If so, how? I've been ordered to add a back up hard drive complete with windows to her computer. How do I manage two operating systems on two different hard drives? Thanks.