Sanjay,
I've tried it on a Windows machine with a fresh Pharo installation and it works fine. The v12 branch of Willow is pointing to the same commit that v12.1.0 tag so this shouldn't be a problem.
Can you describe in more detail the error that you're getting and the steps to reproduce from a fresh Pharo install?
I know that the loading in Windows is really painful specially for projects in filetree format like Seaside. If you don't need the tests you can load the "Deployment" group that pulls less parts of Seaside onto the image so it should be faster.
You can also use the #willow channel in the Pharo discord server or in Buenos Aires Smalltalk Slack (ask me for an invitation if you want) to��ask questions.
Regards
Gabriel