PorBleemo, I would first have to say. ONLY USE THE ADMINISTRATOR ACCOUNT FOR MAINTAINCE AND INSTALL PROGRAMS SYSTEM WIDE. You can make a lot of damage to your computer by using the administrator account for regular day to day basis things such as gaming and web browsing. You probably wan to make you and your friends different accounts that are part of the "Power Users" group, which would allow you to run programs, but is safer and you can't break as many things as with an administrator account.
With Win2k or WinXP you can't restrict users to use a program by date or time. Only the Windows server types of OS allow you to do this. What you want is a third party application that allows you to do this (Idon't know any).
Another way you could try to do it is to change the rights or permissions to that program whenever you don't want your friend using that program. You can probably create a macro to do this, and the use the "System scheduler" to run this macro at specific times of the day to change the permissions of the program to allow/disallow your friend from using those programs.
I hope this works, I am not sure about the second method because I personally don't use windows,
pitupepito