If you have a command line bmp to png converter (or jpg etc. like this one), then you can do this quite easily without any of the complex installs:

#! perl -slw
use strict;
use Win32::GuiTest qw[FindWindowLike SetForegroundWindow SendKeys];
use Win32::Clipboard;

my $clip = Win32::Clipboard->new();

my @windows = FindWindowLike( 0, $ARGV[ 0 ] ); #Find it
die "Couldn't find window '$ARGV[ 0 ]" unless @windows;

for (@windows) {
    SetForegroundWindow( $_ ); # Make it active
    sleep 1;
    ## Send the Alt-print screen key sequence to copy
    ## the active window to the clipboard
    SendKeys("%{PRTSCR}");
}

my $fname = "$ARGV[ 0 ]-@{[ time() ]}";
print $fname;
if( my $bitmap = $clip->GetBitmap() ) {
    local $\;
    my $bmpfile = $fname . '.bmp';
    open my $bmp, '>', $bmpfile or die $!;
    binmode $bmp;
    print $bmp $bitmap;
    close $bmp;
    print "Bitmap written to $bmpfile\n";
    system "bmp2png $bmpfile";
    print "Converted to $fname.png";
}
else {
    die "Couldn't get bitmap from clipboard";
}
__END__
C:\test>screengrab Opera
Opera-1265829469
Bitmap written to Opera-1265829469.bmp
OK      Opera-12658294 ooooooooooooooooooooooooooooooooooooooooooooooo
+ooooo
Converted to Opera-1265829469.png
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