Hi Thorsten,
>> and wouldn't suspect that   it is doing that.
>> Is there some code path I'd trace down to confirm it's actually the
>> virusscanner causing the rename? Where in the code path would the    tmp-file be generated?
> I would first try to use Process Monitor to see activity in the file
> system, one time with and the other time without Avast or in the
> latter case at least with your working copy excluded for Avast. The
> MoveFileEx error should be easy to find using ERROR_FILE_NOT_FOUND and
> from that point you "just" need to scroll upwards to see if files are
> renamed by Avast or such.
>
> Avast will surely not modify Subversions code, but hook Windows
> functions instead and that may be hard to debug. I found Process
> Monitor really helpful in such cases, that's what it's designed for.
>
> http://technet.microsoft.com/en-us/sysinternals/bb896645.aspx
>
> Mit freundlichen Grüßen,
>
> Thorsten Schöning
Should have thought about using process monitor earlier... Thanks for 
the hint.

However, I'm not getting much out there. Here's the summary from what I 
see in process manager from the svn-BDF57639 temp file and the 
corresponding trzBF17.tmp-file which in-deed is created by Avast.
The problem is I don't see where the svn-BDF57639-file would be renamed 
or deleted.

I'd gladly send the log captured with Process Monitor just to make sure 
I didn't overlook anything.

Quick sum-up of the log:
TortoiseSVN creates the file: .svn\tmp\svn-BDF57639 (SUCCESS) - Generic 
Read/Write access - create
[...]
TortoiseSVN reads the file contest of the problematic exe file: 
print_options.exe
[...]
TortoiseSVN writes the contest of that file excluding the final junk to 
svn-BDF57639
TortoiseSVN reads the file contents of the problematic 
print_options.exe-file again (EOF)
[...]
TortoiseSVN writes the last junk of the svn-BDF57639 file and closes the 
file
[...]
Avast creates file access to svn-BDF57639 - Read Attributes/Synchronize, 
Open
Avast queries name information file on svn-BDF57639
Avast closes the svn-BDF57639 file
Avast creates file access to svn-BDF57639 - Read Attributes, Open
Avast operation: FileSystemControl - Control: FSCTL READ FILE USN DATA 
on svn-BDF57639
Avast closes the svn-BDF57639 file
[...]
Avast creates file .svn/tmp/trzBF17.tmp and closes it directly - Generic 
Read, Create
Avast creates file access on .svn/tmp/trzBF17.tmp - Read 
Attributes/Delete, Open
Avast queries attribute tag file - Attributes: ANCI, Reparse Tag: 0x0 on 
trzBF17.tmp-file
Avast sets disposition information file on trzBF17.tmp - Delete: True
Avast closes file access to trzBF17.tmp
[...]
Avast queries directory information on .svn/tmp - svn-BDF57639 is 
returned as existing
[...]
Avast queries tries to open trzBF17.tmp - file not found is returned
Avast recreates the trzBF17.tmp file and writes a junk of 81.920 bytes 
(size of the actual print_options.exe-file)
[...]
Avast queries tmp-directory for svn-BDF57639 - file exists
TortoiseSVN closes file access to print_options.exe
[...]
TortoiseSVN tries to open .svn/tmp/svn-BDF57639 - NAME NOT FOUND

Regards,
Stefan