Over time your computer can load up with rogue start up programs, and erroneous running processes.
Regular virus/malware scans can help, as well as checking your start up programs, and running processes. Look up what they are on Google if necessary, and stop the non vital processes, making sure your not stopping program you need, or something windows needs.
I personally prefer to get my PC to where I like it, then create a complete drive image. That way I can do a quick re-install of windows, than clone over my setup, without spending a day reinstalling stuff.