To Reformat or not to Reformat

My computer has XP pro, is 1000 mghz, 256mb ram, 20gb hard drive.
I have only used 11gb so it is hardly full yet it has become very slow and sluggish.
Even games only requiring 500mghz and 128 mb ram are running slow.
Some suggest I reformat the hard drive some say not to, is this the best move, and how do I go about it?