MSCOMCTL.msm. The common controls funtionality in this package added list and tree view support OS's plus quite a few other enhancements.

I thought list and tree controls were available in Win9x and NT4 and definitely included the updated common controls in an app I built that ran on these OS's, but I forget the exact details. Certainly by Win98SE and Win2K, this was no longer required.

In my app, I used the 40comupd.exe installer package.

I found an 1998 MSJ article that talks a bit about it:

http://www.microsoft.com/msj/0598/c0598.aspx

It's all coming back to me now... (well, not really)