I whipped up a little onscreen keyboard utility for entering katakana and hiragana characters. It's intended for use by anyone (like me)who needs to type simple and occasional Japanese, and can't or won't use the complete Windows IME for this.
Comments are welcome, below or by email, especially from people who are Japanese speakers; I've done my best, but am not 100% certain I got the syntax and Romanizations right.
It's presented in the form of a zip containing a Windows installer; unzip the file anywhere and run setup.exe. You can download it here.
Update: There has been some interest expressed in the source of Kana Put, so I've made it available right here. This was done in Visual C# 2008 Express.