Which version of Windows are you running?  What is the capacity of the hard drive?  How much free space is available on the drive?  (Open Windows Explorer, right click on the icon for the drive, and select properties - this will tell you the capacity and free space). Report back what you find.  If indeed you do not have 15% free space, you will have to get rid of some files on the drive before you can defrag.

Steve

You need to open Windows Explorer (not Internet Explorer), expand the listings for My Computer, by clicking on the + sign, the right click on the icon for the drive [probably -labeled Local Disk (C:)], and select "Properties" from the drop-down menu.  On the properties page for this drive you will see the listings for Used Space, Free Space and Capacity.

Steve

It's not telling me how much free space is available.   I can't find it.  I right clicked on start, properties, no info in there about how much free space.  I searched on internet options, not there.  I have Windows XP.

 OMG!!  I'm with you now.  Sorry about that.  I have hardly anymore free space available.  It shows 1.25 GB available.  I recently added more memory to my drive, why didn't this work?

rose102758,

You cannot add more memory to a drive, the drive's capacity is fixed. If you added more memory (RAM) to your computer that has no effect on your hard drive. If you have 1.25GB free, that represents 15% of an 8.3GB hard drive. I doubt that your hard drive is that small, so to reiterate what volcano11 told you, you must remove items from the hard drive to reach 15% free.