why does my computer stop defragmenting after 12%?

In Windows 95/98/Me, startup programs were the cause of most such failures, but the Disk Defragmenter (Defrag) in Windows XP isn't usually bothered by startup programs running in the background. I would say that there is either a problem with the hard drive itself, or some open or in-use file is getting in its way, or a security tool is preventing Defrag from moving a system file, perhaps because it recognises it as malware.

To check the C: drive deeply enough, run Chkdsk in Thorough mode, To do that open My Computer, right click on the C: drive, and select Properties => Tools. Click Error checking => Check Now... and place check marks in both check boxes by clicking in them with the mouse pointer. Click Start. A message saying that the disk check cannot be performed in normal Windows mode offers to run Chkdsk during a reboot. Choose Yes, and then reboot the PC to allow Chkdsk to run during the startup process. Because the process takes a long time, the best time to do this is when you are not using the PC for several hours, or run it overnight. When the process is finished successfully, it means that the drive itself is all right. Reboot the PC again, and start pressing the F8 key as the startup begins. Press the key quite a few times in order to catch the moment when Windows is looking for that keystroke. The F8 key interrupts the normal boot process and presents you with a boot menu that includes Safe Mode, which you should select. In Safe Mode, Windows XP runs its most basic configuration that makes troubleshooting possible, because none of the higher functions are loaded. If any antivirus or other security programs are running in Safe Mode, disable them temporarily. To run Defrag from the command line, click Start => Run, and enter defrag c: in the Run box. This activates the most basic version of Disk Defragmenter which has no graphical interface. Defrag should now defragment the whole C: drive. Afterwards, reactivate the security programs, and restart the PC.

There are files on the C drive that cannot be defragged while your OS is running, this gets around the problem.

Be certain your Drive has no less than 15-20% loose house earlier than strolling a defrag. Also down load AusLogics Disk Defrag. Just google it, it is loose... you can be competent to peer the defrag in realtime, this manner you'll see it running.

There might be some program interrupting the task, run it in safe mode to make sure nothing writes to the disk while the tool is doing its job. However if you want to continue using the PC while this is going on, you need to switch to one of the automatic tools which work in the background without affecting normal PC operations.

Turn off your screen saver. It kicks in and interrupts Defrag.