Your computer might be infected with virus. Clean your computer from viruses. You can download Microsoft security essentials. It is a free antivirus software and it will clean and protect your computer from viruses, spyware and malware that can otherwise cause your system to run slow. You can also use the Microsoft malicious software removal tool. For malwares, “Malwarebytes” is normally a recommended software in most occasions because it is highly regarded. Whichever antivirus/antimalware software you choose to use ensure you keep it updated.
Defragment your hard disk. This is especially helpful when you add a large number of files, when your free disk space is 15% or less or when you install new programs. To defragment your hard disk, Click start>All Programs>Accessories>System Tools>Disk Defragmenter. There you are meant to select the drive you want to defragment and then click the analyze button. Analysis is meant to show you whether or not to defragment the hard disk.