How can I configure Android to login to an ISA proxy server? 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&rarr;Networks &amp; Wireless&rarr;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 user:pass@proxyserver (e.g. if the proxy is 10.11.12.1, user joe, password foo: joe:foo@10.11.12.1) -- this is how it works in some browsers for password-protected urls. In case this makes trouble (should not, but who knows), further details can be found at Android-Proxy.Blogspot.DE (don't be irritated by .DE, it's in English).

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.