you have running processes which prevent your system from completing the task..do the following.
on your desktop....
click start-then click run-type in msconfig-click ok
a dialog box will appear, click on the ''start-up''tab, then disable all start-up programs, except syst tray
click ok then try it
...or do it the easy way, start your computer in safe mode by pressing the f8 button as it boots up, you will get options showing up,.. one of them is ''run safe mode'' scroll to it and click it..your computer will start in safe mode which disables all your default drives, the help page wil probaley open up as well just close it, you desktop will or should be grey in appearance with safe mode text in each corner, this means your are now in safe mode, now run your defrag, it won't allow you to defrag if you have disc errors, so try running degfrag first if you sit and watch it for 10 minutes or so without getting an alert that says ''your disc has errors and defrag cannot continue until they are repaired'' then go away for 45 minutes , depending on the size of your hd, and it should be finished when you come back.
or if you havent run a full disc scan before then you might as well run a thorough disc scan first[while your in safe mode], do it while your asleep it can take hours , make sure you have the ''fix errors automatically'' box checked then let the computer do it's stuff. then when you wake up your comp will be ready to defrag, plus it will be able to, which is what you want..bye
ps, starting in safe mode requires you to click the ''f8 button'' before windows loads, make sure you click it continually because there is only a small calibration point at which you have to get it to function