I am thinking of installing a alternate login screen with Daniel Milner's Logon Loader (Longhorn Leaf v3, by Cubiz I think), if it all goes wrong what does you computer do when you switch it on? And if above happens, what can you do?

umm you can get a black screen, errors, etc..
I think it ctrl+alt+del press that a few times and it should bring you to the classic logon.
I beilive logonloader as a recovery tool.

usually when the logonui errors out you will get a few popups complianing about it and the classic logon will load on its own

In classic logon will I have to just type my username and password?

*BUMP*

yes
and once your back on the desktop you can change the logon

also im not sure about logon loader but make sure your logon changer can rebuild the logons, what this does is use your oem logon as the base and insert the new images and uifile into a new logonui.exe .. this will prevent any version conflicts that may arrise from useing a logon thats the wrong version and will aslo prevent you from running an executable that could contain virus's trojans etc..