Every Windows version is crappier than the previous one... go figure. Windows7 gets just as slow over time as any Windows version has been done before.
PEBKAC. Windows 7 still runs like a dream on a 3 year old install here. Ditto with XP - i only ever needed to reinstall because of upgrading hardware or because the hard drive with the OS installed on it failed.
Whenever a friend/relative asks me to look at their PC because it's running slowly it's one of the following 99% of the time:
-malware installed by the user
-bunch of harmless crapware(tm) installed by the user that spends an eternity launching at OS startup:
-bloaty/multiple anti-virus/anti-malware programs
-apps that launch in the system tray at OS startup in order to reduce the time taken from the user clicking the icon to the app being ready. Yes they make that app start faster but they hog RAM and significantly increase OS startup time.
-updater tray apps (e.g. adobe reader updater, java updater, etc)
-other unnecessary tray applications (e.g. manufacturer-specific peripheral control panels which are installed with hardware drivers)
Especially with low end PCs that don't have a lot of RAM it's not uncommon to find them with very little or no RAM free at initial startup which causes it have to shuffle the contents between the RAM and the pagefile on the hard drive which is orders of magnitude slower.
-filesystem is badly fragmented
-hardware fault with the hard drive
-other hardware fault, e.g. thermal-throttling CPU