I think the issue is that files names that contain spaces must be wrapped in quotes on MS systems. Have you tried:

my $app = 'C:\"Program Files"\"Microsoft Office"\Office12\excel.exe';

my $arg = "C:\\\"Documents and Settings\"\\my.dir\\\"My Documents\"\\\
+"My Company\"\\QA\\$newFilename";

#Win32::Process::Create( $Win32processObj,$app,$arg,0,NORMAL_PRIORITY_
+CLASS,"." )
#         or die "Aborting... creating process failed due to $!\n";

system("$app $arg");
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Incidentally, if you'd like to avoid some of the excessive use of escaping characters, take a look at quotemeta and/or Quote and Quote like Operators (\Q and \E).