Not sure if the title is specific enough, so here we go:

I crashed my Windows pc, and now I want to install Ubuntu on that computer. I'm currently using my Mac, as it is the only operable computer I have left. I have downloaded the .iso.
I guess my question is; How do I go about installing Ubuntu on my crashed computer from the external harddrive? I've used Ubuntu before, but I used to install it from a CD-rom. I tried reading the instructions, but couldn't really understand them. Please help

Edit: As Tom mentioned, I only want to install Ubuntu on the (formerly) Windows machine. 
Anyways, thanks for the quick responses, I think I've got it from here. Cheers