I use Win 2000 Pro Service Pack 4. Runs everything that I need to including RS Logix 500, PanelBuilder 32, GX Developer, GX IEC Developer, Siemens Step 7 and more. No problems whatsoever.
This all resides on a low spec HP Laptop Pavilion ZE4800 that I bought because it had a built in serial port, PCMCIA slots and was cheap! I’ve spent too long in the past trying to get USB to Serial converters to work with admittedly varying degrees of success. It originally came with XP Home that I removed because of PLC software compatibility problems. 2 1/2 years on with no trouble whatsoever.
Not sure about future compatibility though with new software releases etc, but for now it's perfect and very reliable! I also have another laptop running Win 98 for some old DOS based stuff just in case I need to connect to or modify some old PLC's and message displays etc.
I’m sure that Windows XP Pro is the way to go, but for me at present - if it ain’t broke (yet), then why fix it?
Andrew