wipe my dimension 4600 hard drive

I am going to recycle my old dimension 4600 running xp pro. all i want to do is wipe my hard drive as fast and cheap as possible. How do i do it? i tried reinstalling windows xp from discs that i was given when i purchased the pc, but it wont reinstall because i have an update version of xp pro installed on the pc.

short of throwing the pc in the river how can i wipe my hard drive clean?

Thanks for your help