Could you cite some examples of the poor security? I would rate Windows 7 in the fair or even good category of security. Homegroups now give home users the ability to lock down and encrypt their traffic over their network. Bitlocker provides disk encryption options. UAC can help end users from accidentally clicking on things they shouldn't. Most people find it annoying and disable it, but that's not Microsoft's fault. Built in firewalls have been getting better, and you won't find nearly as many exposed machines on the net because of the Home/Work/Public selection option when you connect to network.
Compared to other versions of Windows, 7 (and even Vista) are leaps beyond what they were before. Samba in Win9x was ridiculously insecure, but all of those holes have been plugged.
It's not very often that I hear about a zero day exploit taking down networks like we had with the worms in the early 2000s. The words "MS Blaster" would strike fear into the hearts of any admin who had to deal with it. I haven't seen anything wreak havoc like it since.
Most network security issues, in my experience, are due to poor network administration rather than holes in the operating system itself. I can't help but feel that the parent comment is a somewhat empty statement.