Mark All unread email as Read - Gmail

I had 4500 unwanted emails and wanted to make my unread email to 0, suddenly i could find a way

1. Go to your Settings/Filters page and create a new filter.
2. In the Has the words field enter “is:unread” (without quotes), and click the Next Step button.
3. You’ll get a message warning you that this type of filter won’t be applied to new mail, but that’s OK.  Click OK to continue.
4. Check the boxes next to Mark as read and Also apply filter to … conversations below.
5. Click Create Filter button, and you’re done.  You might want to should delete the filter once you finish since it won’t be needing it anymore.

Thanks to : http://ragulan.wordpress.com/2009/02/15/mark-all-unread-mail-as-read-gmail-tip/