So I'll admit it, I've still been using PStart until just last month because the pa.c platform launcher was missing some things I really liked. Still is, but I'll bug ya later about that.
Anyway... Didn't notice it until I actually clicked on it today, but "Explore" is bringing up a Windows Explorer window rooted at the *drive* containing the portable apps, but not specifically the directory containing StartPortableApps.exe. For most people perhaps these are one and the same -- but on some of my media, I have the apps one directory lower, in X:\Portable, or sometimes several directories lower (e.g., C:\Documents and Settings\(myname)\Portable, making it convenient to use portable apps within a terminal server environment).
I think that, just like the buttons to bring up Documents and its special subdirs, Explore should bring up the directory just above Documents/PortableApps, even if it's not the root of the drive. (If I want to go right to the drive's root, it's even easier just to hit Win-E and click on the physical drive.)