403 Error on localhost - tough one!

I've searched for quite some time (via Google and many forums) and can't find any answers, so here goes.

I've got a work laptop running XP Pro SP1/IIS 5.1 with a website with ASP pages. There are three situations:
- Connected at work to the LAN, everything works fine.
- Completely disconnected, everything works fine.
- Dialed up to an in-house RAS server, attempts to connect to the website get a 403 error.

I connect to http://localhost/website at each time. PINGs to localhost in all three cases resolve to 127.0.0.1. I've tried loading both ASP pages and HTML pages directly (http://localhost/website/page.asp & http://localhost/website/another.html) and I've also added the slash to the end of the top URL (http://localhost/website/) without success. I DO have admin rights to the laptop, but no CD's are available to me for reinstall of IIS, so I need to figure out the config problem. One thing that almost seemed to work was connecting to http://127.0.0.1/website. We have a sign-on page that is the first to load, which gets loaded this way, but once filled out and the form is POSTed, the page eventually just times out.

Help! Please! I'm at wits end.....

OK, I figured out the localhost issue:

Bypass proxy server for local addresses wasn't checked for the dialup connection - was checked on the LAN (where I kept looking - duh!).

However, still have the same issue as when I did the http://127.0.0.1/website - the login page comes up fine, but when you submit (POST) the form, it just sits there in never-never land....

If you're running antivirus script blocking try turning it off while you're testing your asp pages.

Thx for the reply, Doug!

We have Symantec AV Corp Ed. 8.1.0.825. All the options are grayed out since they don't want us to adjust them, but I don't see anything relating to script blocking anyway. No extra firewall is installed, and the built-in firewall isn't enabled on this DUN connection. I have already put the localhost into the "Intranet" security settings host list in IE since this is an in-house web app and we're using some ActiveX anyway.
Prior to lunch today, I tried to submit the form and it just sat there without timing out. When we got back from lunch (about 35 minutes later), the IE icon was still waving, so I disconnected the DUN connection and viola, the page loaded as it should from the localhost...

Anything else I can look into?

Maybe the DUN setting is changing your gateway and sending all traffic out of your box to the ras server until you disconnect. I suppose a routing expert could verify that for you.