Client-side application and impersonation

Can a browser-based cient-side application impersonate a domain user?

I am trying to host a client-side assembly in a browser. I need to
then impersonate a domain user account in the assembly to give me
access to network resources, and I'm not sure if I'm able to
impersonate a domain account on a client-side application. I've tried
using various samples for impersonation that use the LogonUser API
call in order to get a token, but this fails in my client app, due to
a security exception.

This is an Intranet environment, with the web server running XP. I
have tried running as a trusted site, but no difference.

Apologies in advance if this has gone to the wrong group.

Regards,
Martin
.