Disk errors plague virtually every Windows user at one time or another. These errors can show up in just about any stage of usage. Sometimes this can be something as simple as a bad connection.
As you continue to use your PC, the files get defragmented. The more the defragmentation, the longer it takes time for your operating system to fetch the files resulting in decreased performance. Windows comes with an inbuilt defragmenter which you should run regularly to keep defragmentation levels low.