Network Speed to Windows is Slow

Since upgrading to Mountain Lion my network transfer speed to the Windows 7 computers has become really slow.

With Snow Leopard it is around 80-90MB/s - consstant. I still have a SL boot disk and it's still 80-90MB/s so nothing is changing on the widows side of things.

Same Mac hardware (8 core Mac Pro 3,1) bot in to Mountain lion and I'm getting only 24MB/s.

As far as I can tell, all the network settings are the same.

Any ideas what could be causing this and how I can fix it?

Copying to other Macs from Mountain Lion is still running at 80-90MB/s just like it always was, it's just going to Windows that's slowed down.