How to set default directory in .emacs init file?

[Endless Story]

I don't have this problem running emacs on my Linux desktop - I start
out in the scratch buffer and when I use Find File, it starts looking
in my home directory. I'm OK with that.

But I've just installed gnu emacs for Windows on my XP desktop, and
annoyingly, the Find File command starts looking in the install
directory for emacs! I'd like it to start in my data directory, which
is in a totally separate partition.

Supposedly this is controlled by a variable called default-directory.
However I didn't bring my lisp book with me to where I'm staying at
present, and googling has not produced anything helpful. Is there a
simple way to set this in my .emacs file that will actually work?

I've already tried using Windows itself to set a value for HOME in
System > Advanced > Environmental Variables as my data directory. But
this doesn't work - it just tells emacs where to look for the init
file, not the value of the default directory.