If you are using the built in tool, make sure that no other program runs to interrupt its task,you need atleast 15% free space For this you can uninstall programs that you dont need, burn music, movies pics etc onto DVDs ets, run a diskcleanup(get rid of temp files, recycle bin files, older restore points) and you should be able to get some space free. Run a diskcleanup and chkdsk if needed.Go to Start>Accessories>Systemtools>Diskdefrag...
Analyze the drive regularly and see if the fragmentation levels are safe, do as the report suggests. Just ensure there is no power cut during the process.
Alternatively to save yourself time and the frustration that sometimes arises from defrags that hang or restart, check out trial versions of automatic defragmenters which once installed do the job in the background without needing any manual intervention.