If you are like a lot of people, you notice that your Windows machine becomes slower over time.  This is due to your constant install and uninstall, include the multiple updates that your softwares get.   Even if you don't install new software, you find that your Win machine grows slower over time.  You install a cleaner program like CCleaner, and sometimes it improves your system by removing orphaned and unnecessary programs from your system, but if you are not careful, you might break your system if you remove a needed dll, for example.   Sometimes, the only way to ultimately clean your system is to do a full reinstall of WinXP.