You need a few things:
1. Your Sharepoint Site needs to be configured for Challenge/Response Authentication (the default)
2. You really should have the site protected at all times by an SSL cert – see our friends QuoVadis for one right now.
3. Add your portal site to your ‘Local Intranet’ site security settings in IE. This has the option ‘Automatic Logon Only In Intranet Zone’ which will pass your credentials to this trusted website.
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