Figured out why I was getting 403 errors when I was trying to go to Archives from my main page, and I don't think it was posted yet.
Check to see that you do not have a physical directory structure that matches your permalink structure.
Like if you have permalinks that look like 2005/05/blah-blah-blah, don't have a physical directory structure 2005/05/other-stuff or whatever.
I had moved over from Blogger.com to WordPress, and I had published my Blogger blog a few times. My directory structure for the published Blogger blog matched the permalink structure I was trying to use in WordPress. After I re-named the directory with the Blogger posts, I could access the WordPress archives like a charm again.
Before finding this out, I was able to access my 2005/06 archives just fine, but not my 2005/05 archives. Well, I had switched over to WordPress before June, so I hadn't published anything in Blogger in June.
About all I can say is that something "got confused" and out popped a 403 error.
Maybe someone else can offer a more technical explanation.