Whenever you install any applications, always always always look for a custom install option and then uncheck all the bloatware.
Many people think this only applies to shareware, but fail to notice that even brand name applications install all kinds of extra junk from an automatic install. Ditto driver packages for peripherals you use.
Uncheck things like automatic updaters, task tray monitors, unneeded 'helper' apps, toolbars, daemons, add-ons, etc. etc. All of these things combined start dragging a system down, and most of it is totally unnecessary. Just a couple of applications (say, Office, a disk burning suite, iTunes, and antivirus) installed on autopilot can totally pollute a machine.
I like to keep my machines running lean and clean. To me, an application should run only when I need it, and afterwards closed and not using system resources. If an application really needs a system process constantly running, then I'd really have to weigh my need for it on my system.
So keep an eye on what gets installed, and check the taskmanager for what manages to slip by.