There are many ways by which your system gets slow down but all the problems have common remedies; the following are the ways by which we improve our system speed.
1. The disk clean operation must be done regularly. This option is found when you right click on any drives and go to properties option.
2. Disk Defragmenter is necessary. Go to that option as shown in the picture and right click on drives to defragment them.
3. Cleaning of Registry. We can't manually clean the Registries so we can use any soft wares as shown in figure.
4. You improve your start up speed by removing all unwanted soft wares from start up. You go the dialog by typing msconfig run option available in start.
5. At last keep your system free from all viruses by installing a proper antivirus of original version do remember to update the antivirus for better results.