How to clear my laptop.?

Hi guys, some help please, my dearly beloved has bought me a nice shiny bright brand new laptop, which is fab, now I need to sell on my old one which is an Acer running on Vista it is about four years old. Now I am no scientist so the easier the better please How do I completely wipe it clean, all memory wiped and all Browsing history Etc.I want to be sure it is done completely as I have spent the last four years on it researching my family and doing my household Banking and finance etc.I have already transferred anything I wanted so I just want to p it on and hope someone can get some use out of it. Many thanks .....