According to the MSDN Library doc, the Win32_Volume class is not available on XP or prior windows versions. Could that be your problem?

Here's code I typically use... (that I haven't used since 2K and could be flaky on XP)

my $class = "Win32_LogicalDisk";
my $Info = $SWbemServices->InstancesOf( $class )
    || die "Could not access instances of $class on $hostname";
for my $Item (in ($Info) ) {
  if ( $Item->{DriveType} == 3 ) {
    my $size = int( $Item->{Size} / (1024 * 1024 * 1024) );
    my $free = int( $Item->{FreeSpace} / (1024 * 1024 * 1024) );
    print $Item->{Name} . " $size GB\n";
  }
}
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