How to clean off my hard drive without a floppy disc

I have a Dell Dimension 4600 that has a problem and have tried everything to fix it. I was told to clean off my hard drive and re-install everything to fix it but all the directions I find to do this require downloading onto a floppy which I do not have. I need instructions in simple English please as I am not a computer whiz on how to clean my hard drive and start fresh to hopefully fix this mess I am in..