It is SSO (single sign on) related. You may consider:
1. LDAP, ADS.
2. NT Authenticaion.
3. Customized SSO, like the hidden form.
Regarding #3, you could auto submit the hidden form, then in the handler page to read the form variable and put it in session. ( There are two sessions, ASP session and CSP session.) During the processing, you may consider using encrypt / decrypt the user id and password in the hidden form.
Good luck,
Ben