That was my first thought as well
A slow down in performance can indicate viruses and spyware are rampant on a machine.
You've already mentioned toolbars and messenger clients, and these are all too often the source of malware. Young people see a fascination in toolbars for some reason, and many of the toolbars spy on internet habits to target searches and advertising. IM's can be particularly dangerous, especially if they are used in unmoderated or suspect chat rooms, and if the users transfer files between each other without virus checking the files they receive.
On a network especially, you need to be well armed with firewalls, anti-virus applications and anti-spyware utilities, and set up a regime of regular scanning.
Besides that, I believe your quest for leaner machines should be concentrated into 3 areas;
Whenever any application is installed, ensure it does not set itself to start when the computer is turned on, unless it is a necessary, eg anti-malware type, process.
Msconfig will help you remove any applications that load on start up in the Startup tab. Look for Instant Messengers, (these can be opened when required), Media Players, (ditto), especially Real Player, addons and extras that are really not necessary.
Bob's list will help you decide which are which in MSconfig. Also, the Services console will give you more descriptions of each service, (Start > Run, type in services.msc then click OK). Services can be set to automatic, set to manual, or disabled in the console.
The Processes tab of the Task Manager, (right click the Taskbar, select Task manager), is a great source of information for anything that is running that shouldn't be there, like viruses, spyware, etc.
This site, http://www.answersthatwork.com/Tasklist_pages/tasklist.htm will help you decide which are critical, useful, or downright dangerous.
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