(D) Windows XP Instructions:
In WinXP, the word SCANDISK has mysteriously disappeared and been replaced by the words CHECK, ANALYZE and/or CHKDSK (the latter being a throwback to pre-Windows DOS commands). In any case, they all mean your hard disk will be "analyzed" before you run DEFRAG.
Go to Start, All Programs, Accessories, System Tools, Disk Defragmenter. Click the Analyze button, and then click Defragment after the analyzing has been completed. However, if you click the Defragment button first, you'll be told your disk is being "analyzed," after which defragmenting will begin.
Another approach is to go to Start, My Computer, and right-click Local Disk C. Click Properties, Tools and you'll find buttons for Check Now and Defragment Now. Having done this, you may be told that these programs can't run without rebooting your computer, and asked if you'd like to run them the next time you restart.
If this all sounds rather convoluted, there is yet another approach. You can go to Start, Run and type CHKDSK. Click OK and WinXP's equivalent to SCANDISK will commence in a DOS window