Windows 7 Tip: 
Improve performance of your PC by defragmenting your hard disk, Disk Defrag is simple application which allows users to defragment their HDD space. It optimizes Windows Disk Defragmenter by running it under optimal conditions. It restarts you computer and then runs only defragmentation. Once defrag is over, it automatically shutdown or restart your computer.
See Also: 6 ways to speed up your PC Performance
It is also helpful when you want to run defrag before you go to sleep, disk defrag will simply shutdown or restart your computer after defrag is over as per your instructions. Not the least, you can select multiple drives to be defragmented at once.
To defrag you need to follow simple steps -
# Select the drives to be defragmented.
# Select the defrag option as per your need.
# Select if you want defrag to proceed if free space is low.
# Simply click defrag and you are done.
Download Disk Defrag here
Just use Diskeeper to defrag the drives automatically, its very easy and efficient. Set it on background mode and go about work on the PC. It defrags without skipping any files, even under low free space. Also great to defrag system files.