Access to network restricted for certain user?

Hello,

is it possible to restrict access to the network for certain users?

I have a network / security problem with a sharepoint server. From
sharepoints point of view, my web part code is allowed to access a
certain url, but I get timeouts if I try to access the url.

If I try to access urls I've not configured as allowed, I get the
expected security exceptions, so the security settings in sharepoint
seem to be ok.

If I login as admin, I can access the urls via a web browser, so
networking seems to be ok also.

But: What causes the code in sharepoint to timeout? I checked the server
on the other side. There is no request incoming at all.

I'm a developer, not an admin. I have no idea where the problem might
come from an how to debug it.

It's a Windows 2003 server with sharepoint installed. Any hint would be
very appreciated.

regards,
Achim