Easy Fixes for Slow Computers: 3. Defragment your hard disk drive.
What is fragmentation? I’m not a techie, and this is the best definition I found:
“Fragmentation describes the condition of your hard disk drive when files are divided into pieces and scattered around the disk. Fragmentation occurs naturally when you use a disk frequently, just by doing basic tasks like creating, deleting and modifying files.” (Source: Webopedia.com)
Microsoft.com features a helpful video and step-by-step instructions for how to defragment your hard disk here.