There could be a big number of reasons why this is happening. From faulty hardware installed on your computer to Windows operating system with all its flaws, viruses, bad drivers, etc.
You could try first and uninstall applications you don't need. You can do this by visiting Add or Remove programs found in Control Panel.
Or, you can use a tool like CCleaner to clear all your unnecessary registry entries, temporary files and manage all your startup entries.
