Why Windows Slows Down Over Time
Startup Programs: Examine a Windows system that’s slowed down and you’ll likely to find many additional startup programs have been installed, lengthening the boot process, cluttering the system tray with useless icons, and consuming CPU, memory, and other system resources in the background. Worse yet, some computers may come with a huge amount of useless startup programs out of the box thanks to manufacturer-installed bloatware.
Explorer Plug-ins, Services, and More: Applications that add shortcuts to Windows Explorer’s context menu can make right-clicking on files take much longer if they’re badly programmed. Other programs may install themselves as a system service, so they’re running in the background even though you can’t see them. Even if they aren’t in the system tray, useless programs can slow down your PC.
Heavy Security Suites: Security suites like Norton are often very heavy, consuming a lot of resources to perform all their functions. You don’t really need a full security suite — just an antivirus program.
PC Cleaning Tools: PC cleaning tools are generally scams. Paradoxically, they can make your computer even slower if they add themselves as a startup program and run in the background. The scammiest PC cleaning programs may even install additional spyware and other junk. Use the free CCleaner instead of paid PC cleaning tools.
Other Junk: Poorly written applications may clutter your system with useless DLL files and fill your registry with useless entries. The worst applications may not clean up properly after themselves, leaving this stuff on your system even after you uninstall them.
Browser Toolbars: Legitimate browser extensions can slow down your browser enough, but junk add-ons like the terrible Ask.com toolbar can slow things down even more.
In other words, the main cause of a Windows system slowing down over time is installing junk software.