Vista - ASP.NET ajax client side framework failed to load

Hello,

I am running a website which receives the following error whenever I
run it:

"ASP.NET ajax client side framework failed to load"

I am using .NET 3.5 and have the Ajax toolkit installed (the correct
version).

I have already checked my web.config file and everything looks good
(correct sections, handlers, ....).
I also checked whether my scriptmanager is correctly placed and it is
placed directly under my form-tag in the masterpage, so it can't e any
higher in the page. I also added scriptmanagerproxies on my
contentpages. I tried different things with the toolkitscriptmanager
in different places, but none of them worked.

We are working with other developers and they can run the website on
their pc's with the same configuration (IIS 5.1, ASP.NET 3.5, Ajax
Toolkit, Windows XP Pro)

Another thing that I have noticed is that when I use SSL the error
dissapears. Which is is quite strange I think. Obviously this is not a
viable solution as I don't want to put my entire web site under SSL.

Does anyone know anything else that I can check?