Hi, Hans —
If you just want to lock the lefthand nav (the "Main Menu" tiddler) in place, add this to your StyleSheet tiddler:
/*{{{*/
#mainMenu { position: fixed; }
/*}}}*/
If StyleSheet doesn't already exist, you can create it and add the bit above.
You can also lock the sidebar in place by adding this line:
#sidebar { position: fixed; }
The header is a bit difficult to lock in place because of some additional characteristics it has (namely the background color gradient, which is done with a TiddlyWiki core macro). It could be done, but you'd have to dig into the CSS a little more intensively.