When you buy a new PC the chances are there are a load of programs pre-installed onto it. Old PCs can simply accumulate these over time. Most of these you'll never use and some of them can often run in the background without you knowing, slowing your computer down in the process.
To remove these click "Start" then go to "Control Panel" then click "Programs and Features" and scroll through the list, uninstalling the ones you never use.
If you're unsure what should stay or go, use PC Decrapifier - a free piece of software that recommends what is worthwhile or not.