WTF?

Windows is pretty ridiculous for a processing cluster, the whole premise of windows was that it's a gui (hence the name)... It requires a videocard to boot, and will not support a serial console...

Why would you want all that extra hardware in your cluster nodes? Boot everything headless, redirect the console over a serial connection for debugging...

Also being closed source, you can't tune the software to your individual needs.

Also the costs, once you have a large cluster you want each node to be as cheap and power efficient as possible - so you can afford more nodes... You don't want to waste money on software for each node, and you don't want surplus hardware that you had to pay for, and which will consume power which you also have to pay for, and produce heat which you have to pay to get rid of...

You want each node to be diskless and network boot, have as much ram and cpu cores as is optimal for your workload, have whatever interconnect is necessary and nothing else. No video hardware, no onboard sound, no usb, no ps/2, nothing that will waste power.

Just look at the current market leader in the top500 supercomputers list - IBM.