i would suggest you perform these steps:

=> Open My Computer
=> Right click on Drive C, click Properties
=> On the General tab, click Disk Cleanup
=> Allow the Disk Cleanup process to finish, it might take at least 30minutes to an hour (depending on how large is your drive, and how long it has been since your last disk cleanup)
Right click again on Drive C, click Properties
Click on the Tools tab, click Defragment now
Allow the defragment process to complete. This will also take some time depending on the size of your drive and how long it has been since your last defragment.