Access user credentials with impersonate=admin acct?

[Burt]

I've got an intranet app using integrated authentication that
impersonates a admin account in the web.config. However in one part of
the app I need the user's credentials to access a web service (it's
pulling back a MS Reporting Services report, and only certain users can
view certain reports).

If I pass in CredentialCache.DefaultCredentials, I get the impersonated
account. I could System.Net.NetworkCredential("user", "password",
"domain"), but how would I get the user's password?

Can anyone tell me what I'm missing?

Many thanks,

Burt