Need help reformating my comp!

[Broken Comp Man]

I have a dell dimension 8400 series, its not working properly so I wnat to reformat it but don't know how
I dont have any discs for it anymore.

 

[softe]

i think it would help if you had a windows XP or any other OS, you need to buy one first then if you need help with the installation, post it on this forum.