1. Delete the temporary internet files regularly.
2. Defragment your hard drive once every two months. This will free up a lot of space on your hard drive and rearrange the files so that your applications run faster.
3. When installing new Softwares disable the option of having a tray icon. The tray icons use up available RAM, and also slow down the booting of your PC. Also disable the option of starting the application automatically when the PC boots. You can disable these options later on also from the Tools or preferences menu in your application.
4. Refresh the desktop after closing any application. This will remove any unused files from the RAM.
5. Empty the recycle bin regularly. The files are not really deleted from your hard drive until you empty the recycle bin.