If anyone doesn't know how to see this, here's how you do it:
Start > Control Panel > Performance and Maintenance > Rearrange items on your hard disk to make programs run faster > Click Drive > Analyze.
These images are baring in mind I have pretty much never defragmented my hard disks before.
Here's my C drive. This is the drive I use for only my OS and Word Documents, that sort of thing:
Now here's my D drive. This is the drive I use for downloads. Pictures, Music, Games, TV Shows, Movies, etc go in here. As you can see it is a 233GB drive and I only have 2.8GB left.
I am defragmenting them tonight.