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If you have 404 errors listed on your report it is because somewhere on your page is a link to a url that is either mal-written or no longer exists. You can export the data from your report or run a manual crawl using the research tools and the 404 error will be listed along with the referrer (the page the broken link is on). Go to that page and either correct or remove the link.

Thanks doford again for your answer, yes that makes sense. I wonder if i can pick your brains on this. I did make another post here on this.. http://www.seomoz.org/q/weird-404-error you can remove mywebsite.com and replace it with mine if that helps? Regards