Site Showing 404 Errors – Not Found

I hope someone can help me as I’m totally baffled over this. In the Google Webmaster tools it’s showing that I have 297 404 Errors – Not Found in my site. I have no clue how this happened! It gives me the list of urls and when I click in each of them it shows the Search box on the page (which I don’t use in my site) and above it it reads 404 Error – Not Found. It has the header and the footer but the page is blank with just that search box.

I have looked everywhere to try to find these pages/posts, etc. to remove them but they are nowhere to be found. Obviously if Google found them, they have to be buried in my site somewhere.

Can someone please help me and tell me where I can find all these pages and remove them? This is killing my rankings.

Thanks!