Why are these cookies on my computer?

Recently I was advised to delete all cookies, amongst other things, as this would speed up my slow computer. I started to wonder what cookies were being placed on my computer and how much they slow my computer down.

When I log on cookies from various companies are placed on my computer. These are not just from Accounting Web, but from others as well. They are :-

www.sitestat.com
www.doubleclick.net
www.sift.co.uk
www.atdmt.com

I thought that Accounting Web only placed their own cookies on my computer. Why are these here?
David Honneyman