I don't think they're caused by excel.

Some background:

When excel saves the file, it saves it as a temporary file with a funny name (8
characters--no extension) in the same folder.  

If the save is successful, xl will delete the original (or rename it to its
backup name (like "backup of book1.xlk)) and if that's successful, xl will
rename the funny named file to the original's name.

If you're seeing that funny named file, then something is going wrong.

Common things that get blamed for interruptions to this process are antivirus
software poking its head in or network errors--either permissions or physical
problems.

Saved from a previous post--Your file names look close to the AVG suggestion:
==========

If those .tmp files look like:  ...\VB*.tmp

I searched google again for "\vb" and .tmp and got a few hits.

One of them:
http://groups.google.co.uk/group/microsoft.public.excel.programming/browse_frm/t
hread/2d87fd16570374f/061eecaad1c774cc?lnk=st&q=&rnum=3#061eecaad1c774cc
or
http://snipurl.com/1339g

One person posted that they saw files that had names like:  ~RF26c83af.TMP.

It was caused by AVG.

You can read her description in this thread:
http://groups.google.co.uk/group/microsoft.public.excel/browse_frm/thread/4fbfd7
18699c49a4/cccd4eed0638e395?lnk=st&q=group%3A*excel*+insubject%3A%22Duplicate+in
subject%3Afiles+insubject%3Aopen+insubject%3Afrom+insubject%3Aautorecover%22&rnu
m=2&hl=en#cccd4eed0638e395

or
http://snipurl.com/jyus

A while ago, someone posted back that uninstalling something called "Panda
ClientShield" fixed a similar problem.

Even if you're not running "Panda ClientShield", you could try disabling this
program to see if the problem goes away.