http://www.engadget.com/2007/01/29/how-to-...-running-vista/
QUOTE
1. Download the firefox extension for spoofing user-agents: https://addons.mozilla.org/firefox/59/
2. Restart firefox and goto: Tools > User Agent Switcher > Options > Options...
3. Click "User Agents" and then "Add"
4. Fill out the window with following info:
Description: Internet Explorer 7 (Windows Vista)
User Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0)
App Name: Microsoft Internet Explorer
App Version: 4.0 (compatible; MSIE 7.0; Windows NT 6.0)
Platform: Win32
5. Save it, goto Tools > User Agent Switcher > Then click the newly added one. Browse over to http://hotspot.t-mobile.com/vista/ and you should be redirected to a trial sign-up page.
2. Restart firefox and goto: Tools > User Agent Switcher > Options > Options...
3. Click "User Agents" and then "Add"
4. Fill out the window with following info:
Description: Internet Explorer 7 (Windows Vista)
User Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0)
App Name: Microsoft Internet Explorer
App Version: 4.0 (compatible; MSIE 7.0; Windows NT 6.0)
Platform: Win32
5. Save it, goto Tools > User Agent Switcher > Then click the newly added one. Browse over to http://hotspot.t-mobile.com/vista/ and you should be redirected to a trial sign-up page.
Haven't tried it yet to see if it works. But another comment makes it seem it is not possible.. If someone wants to try, please report back if it works or not.
QUOTE
This article is entirely false. I'm sitting at a T-Mobil Hotspot right now, running Windows Vista. You may be able to sign-up by spoofing the user agent, but once you get to the hotspot, it makes you install an ActiveX control that does other Vista checks.