Umihara Kawase-san Saver
Umihara Kawase-san Saver (海腹川背さんセーバー) is a Windows screensaver which will cover your screen in Kawase. Many small Kawase will walk across your screen, and if two of them bump into each other, stars will fly out of them, and their silhouettes will be left on the screen where they were shortly after they bumped. Most of the Kawase you'll see on your screen will be wearing her Umihara Kawase Shun outfit, but you'll occasionally see one in her original Umihara Kawase outfit.
Since the screensaver was released while the latest Windows version was XP, the download site only specifies compatibility with Windows 98, 2000, Me and XP, but my testing has confirmed that it still works fine as of Windows 8.1. The readme file says it should also work in Windows 95 and NT, but wasn't tested with those versions.