Depending on the Complexity. theres a few ways to do it.
to most important decision you must make before hand is.
Price. and Complexity.
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The Windows Route -
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Windows is usually not you're best bet for small businesses because of cost of software.
in order to track what "users" are online, you have to have a way of managing each PC and tracking the usage.
This means a Server and Windows Domain.
if all you're computers are using windows 98 to Windows XP(not including home edition)
then you can use either Windows 2000 Advanced Server or Windows Server 2003.
Both have a user interface to set it up and are rather simple to do.
then just add the PC's to the Domain, and setup users on the server.
Everytime some one wants to buy time, just give them a temporary username,(3 for each system, assigned on the server. for theres no confusion)
and just set account to disable at the conclusion of there payed time. it, you can manage and see whos logged in currently using the built in management software. and see how long theyve been logged in and what applications they have open.
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The linux Route:
In the same sense as windows, Linux has lots of options for managing PC's. without the Cost of windows, and with more options...
The only problem is Configuration and Setting it up. Depending on you knowledge of PC's and youre Familiarity of linux. you should probaly hire some one to set this type of management up.
Effectively it would be better to use linux, and any way you look at it, cheaper. in my personal opinion, it has alot more applications to choose from as well. but ultimately its up to you.
As far as the actual management, you you beleive you can set it up youre self....
you will need a linux Distribution with atleast kernel 2.6.16, installed with Xorg, TCP/ip, ldap, kerbose and set up the system for shared logins.
from there how you set up the systems is up to you. i would consult the web for more options.