Git problems with adding files (first use)

Dear all,

I have just installed Git from the msysgit distribution
(Git-1.6.4-preview20090730.exe).  I've also installed TortoiseGit
(1.0.2.0).  I've been having a few problems adding files (either using
tortoise or the command line tools).

What I did (copied and pasted directly from the command window):

=============================================================
Z:\>mkdir testproject
Z:\>cd testproject
Z:\testproject>echo "test file" > file1.c
Z:\testproject>echo "test file" > file2.c
Z:\testproject>git init
Initialized empty Git repository in Z:/testproject/.git/
Z:\testproject>git add file1.c
error: unable to create temporary sha1 filename .git/objects/f7/tmp_obj_a02340:
No such file or directory

error: file1.c: failed to insert into database
error: unable to index file file1.c
fatal: adding files failed
=============================================================

Can anyone tell me what I'm doing wrong?  I've tried using tortoise
git and I've tried it on an existing project (rather than a bunch of
random files), but I can't seem to add files to the repository: I
always get the "No such file or directory" error.

Thanks in advance for your help.

Al