Typically that happens when Excel shuts down abnormally, often as a
result of the program crashing, being stopped wtih CTRL ALT DELETE or
via TaskMgr, or the computer being shut down while Excel is open. As
often as not, you need to restart Windows to clear the lock on the
file.
Cordially,
Chip Pearson
Microsoft MVP 1998 - 2010
Pearson Software Consulting, LLC www.cpearson.com
[email on web site]