1) as already said by the other posters this is no problem. It is easier to do it on a 2nd hd but the installer even has an option to let ubuntu autoresize the windows partition and install itself on the new partition created from the free space.
2) choice galore here, webbrowsers: firefox, konqueror, opera, ...
mail clients: evolution, thunderbird, Kmail/Kontact, mutt, pine, ...
3) The only true wysiwyg is NVU but indeed a bit unstable. Bluefish and Quanta are not really wysiwyg, but have loads of options for code generation, code completion, previews, etc. You can also use VI or Emacs with their respective html-plugins. Wysiwyg usually leads to ugly code with a bad distinction between contact and markup anyway.
btw you can also try Kubuntu, which is Ubuntu with the KDE desktop as default. It is a little more windowslike and might ease the transition a bit. Be carefull though, the desktop wars are a big thing in the linuxworld
