One of the things that will slow down your PC, is the Windows registry. What is the windows registry? To be short, it is something like the ledger that contains the information off all programs that have been installed on your PC. It also contains a set of values that your PC needs to handle certain stuff like handling your Internet connection. Now while you are using your PC over time, there will be stuff in this registry that becomes obsolete or invalid. This is due to uninstalling programs that still leave traces behind in the registry, for example.
Another thing that slows down your PC and gives you lag, are the traces that are left behind while surfing the Internet. With these traces of stuff like cookies and Internet surfing history, your Internet connection often gets slower and slower. Checking and cleaning the traces of your browser on your PC, as well as cleaning your registry can be easily done with one tool called ccleaner(http://www.piriform.com/ccleaner). When you have downloaded program, just open it up after installing and let it run on the first tab called "cleaner" with its default settings. After the program has cleaned all of the windows' traces, you go down one tab, which is called register, and let it check for errors in your windows registry. When it has scanned the windows registry, press on the button to fix the selected problems. When it asks you to make a backup, you only need to do so for the first time. This is to make sure you still have restore point just in case you might have deleted something you still need. If that is the case, you can just use ccleaner to restore the previous register values. If you have scanned the first time and you didn't have any problems afterwards, you do not need to make a
restore point the second time you scan it.
Another point of focus, is the fragmentation rate of your hard drives. The more fragmented your drives are, the slower your windows will find the files it needs for the requested operations. Windows has an inbuilt tool for this but since windows vista, this tool isnt quite as good as it was in my opinion. Instead, I prefer to use Ultimate disktrix defrag (http://www.pcworld.com/product/950263/ultimatedefrag-free-public-domain-edition.html). It has some advanced options, such as placing more used files on the inner ring of your hard drive so that they will have smaller loading times as the files are found and read faster. Its usually a good idea to defragment after installing or uninstalling programs.
I will update this article as i find or remember more useful tools that help in cleaning and maintaining your PC. In the meantime, I hope this helped you :).