You can still use html but use a technice called remote scripting.Check out
msdn website:

[L=http://msdn.microsoft.com/library/default.asp?url=/library/en-us/rmscpt/Html/
rmscpt.asp]http://msdn.microsoft.com/library/default.asp?url=/library/en-us/rmsc
pt/Html/rmscpt.asp[/L]
Short expaination:
U Use a Iframe or Javaapplet on your page and size it as small as possible.
This frame u can use as a small container to load content so that u dont have
to reload the hole page.
To get the loaded content in the frame u use javascript to update the
dropdownboxex etc.
//Criss