I am successfully using an older PortableApps.com version of Dia Portable v0.97 with Win7. Downloading and trying to run the latest 0.97.1-1 fails to open.
After installing, when trying to run, a few boxes open, many with no text.
Dia Warning (title of window) no text inside
diaw.exe has stopped working (window)
Diagram1.dia (window) blank white screen
reverting to the older 0.97 version works just fine.
I have not tried it under a different OS.