On the Webstats page there is a list which includes 401 - page not found. I noted there was a significant number for my site and spent some time going through the pages looking for missing links. I found a few and corrected them. It would be useful to know what generated these 401s. Is there any chance that Webstats could show a list of what links generated the 401s? If they were internal they can be fixed; if they are external links they can be checked out (site may have gone) and amended or delted as appropriate. There is nothing worse than a site with dead links.
There is a rise in script kiddie attempts to go to common subURLs used by blog, forum and gallery software. In searching for these pages, they hope to find an old, vunerable relaese of one of these pages.
Other than that, W3C have a nice online tool for testing your site for broken links at http://validator.w3.org/checklink. A search for 'link tester' in a serach engine will show some freeware programs out there too.