OK C Stop.
Ok. Is it windows XP? If so put in your windows XP disk, and boot from this.
Look at what the screen is saying when it comes up with press "R" to repair press it. It will load up repair console. and should give a prompt.
If your main Administrator account has a password you will need to enter this.

When a prompt is returns type CHKDSK /R

May take ages.
There could be an underlying issue ie bad ram/overheating CPU or HD going bad which is currupting data. Or all the porn you download has taken its toll.