I see that if I did a long touch over a Wifi network, I can specify some proxy settings. In my company we use a Isa Proxy server, and each user has to login through domain server.
Is there any way to configure that on our tablets? ( Android 4.1)
There are multiple solutions. As you have Android 4.x, long-pressing on a WiFi APN in Settings→Networks & Wireless→WiFi lets you edit an APN. Check the "Advanced" box, and you can configure the proxy. Though this form does not offer separate fields for username/password, here's a trick how to do it:
For the server name, enter
Another possibility would require your device to be rooted: ProxyDroid offers some comfort in connection with proxies. Next to supporting multiple profiles, it is also capable of handling different authentication methods.