Weird 404 Error

I have set up WPMU and everything seems to be working fine except for a weird inconsistency when I click on the Create a New Blog link.

1. On my work computer I get a 404 page not found error, although sometimes when I continue on to another web page and then hit the back button with my browser I can see the page (that previously displayed 'page not found').

2. But on my home computer when I click on Create a New Blog I can see the new create a new blog page although in the top of the browser it says 'Page not found' next to the name of the Blog.

3. Both situations happen in both Firefox and IE

4. However, when I add a new blog from within the main admin site everything works fine.

Can you give me some direction on how to correct this?

Don