When I was using my win98 computer, I can't access to AGS site.
Is it because my internet on win98 was slow (around 66k/s) or all my browsers were outdated?
Well the site uses CSS2 for layout, which started to be fully supported about the time Win98 was released. So unless Peder has the server configures to forbid super old browsers from access it (which is what 403 errors can mean, as well as your trying to access things you're not meant to, and scripts going a bit weird), I have no idea. Since I don't think Windows 98 will run most AGS games very well anyway, it's not really something there's much reason to support anymore...
I just checked which browsers I'm using on win98.
Opera 10.10
Firefox 2.0.0.20
Internet Explorer 6.0
Are those browsers considered outdated and getting 403 error?