I agree, biggest reason why windows slow down is many software you install add autoupdate or helper functions that run at startup, and gradually leech life out of windows. Printer drivers are a big culprit. You can use CCleaner, AutoRuns or even built-in msconfig, together with information at http://www.sysinfo.org/startuplist.php,
to review list of startup items to disable.
I review startup items religiously every time I install new software or an upgrade.