Try it in safe mode, if it works in safe mode, it may be something on startup that is causing it to slow down. Also try running checkdisk if safe mode does work.
To use safemode, turn off the computer. Turn on the computer and keep tapping F8. A menu should appear and then you can stop tapping F8. Highlight the first Safe Mode option and press enter. It will boot into safe mode.
To run checkdisk, open command prompt (It should be in the start menu, or press Windows+R and type cmd, then press enter) and type "chkdsk <Drive>: /f /r" where <Drive> is the letter assigned to your drive. It may ask if you want to check the disk at the next boot, type Y and press enter if it does. It will take a while to scan and repair your drive, but once this is done, it may speed up your computer again. If not, return here for more advice.