Just know that you're not alone. 12.3.1 has been a total disaster for many, apparently on both Mac and Windows.
Itunes not responding, running extremely slow
It's been a month and a half and Apple refuses to even confirm that they broke something in the 12.3.1 update (meaning don't expect Apple support to be of any help on this - they'll send you down 10 different time-wasting rat holes, knowing full well these won't fix what the iTunes team broke), much less fix it (and when they do finally fix it, it will be with no acknowledgement of all the time and frustration caused by their total galling unresponsiveness to the crippling bug that they injected into their own ecosystem-core application).
The only viable solution that I have read involves downloading an earlier version of iTunes from an unverified sources, deleting files via Terminal, installing the unverified, non-current version of this critical software and hoping nothing breaks during the downgrade, and then disabling auto-update to prevent both a) the sure return of a known, verified, repeatable, major bug, and b) any ability to take advantage of future iTunes updates if a real fix isn't addressed and incorporated in the iTunes code base going forward. And that's on the Mac side - not quite sure how that mess of a hack would even translate over to Windows.