In the initial designs for the Start menu, the list of most-frequently-used programs on the Start menu would be completely empty the first time you opened it. This was perfectly logical, since you hadn't run any programs at all yet, so nothing was frequently-used because nothing had been used at all! Perfectly logical and completely stupid-looking.
Imagine the disappointment of people who just bought a computer. They unpack it, plug everything in, turn it on, everything looks great. Then they open the Start menu to start using their computer and they get... a blank white space. "Ha-ha! This computer can't do anything! You should have bought a Mac!"
(In usability-speak, this is known as "the cliff": You're setting up a new computer, everything looks like it's going great, and then... you're staring at a blank screen and have no idea what to do next. The learning curve has turned into a precipice.)
The original design attempted to make this initially-blank Start menu less stark by adding text that said, roughly, "Hey, sure, this space is blank right now, but as you run programs, they will show up here, trust me."
Great work there. Now it's not stupid any more. Now it's stupid and ugly.
It took a few months to figure out how to solve this problem, and ultimately we decided upon what you see in Windows XP: For brand new users, we create some "artificial points" so that the initial Start menu has a sampling of fun programs on it. The number of artificial points is carefully chosen so that they are enough points to get the programs onto the Start menu's front page, but not so many points that they overwhelm the "real" points earned by programs users run themselves. (I believe the values were chosen so that a user needs to run a program only twice to get it onto the front page on the first day.)
Note that these "artificial points" are not given if the user was upgraded from Windows 2000. In that case, the points that the Windows 2000 Start menu used for Intellimenus were used to seed the Windows XP point system. In that way, the front page of the Start menu for an upgraded uses already reflects the programs that the user ran most often on Windows 2000.
In the initial release of Windows XP, the "artificial points" were assigned so that the first three of the six slots on the most-frequently-used programs list were chosen by Windows and the last three by the computer manufacturer. If your copy of Windows XP was purchased at retail instead of preinstalled by the computer manufacturer, or if the computer manufacturer declined to take advantage of the three slots offered to it (something that never happened in practice), then Windows took two of the three slots that had been offered to the computer manufacturer, leaving the last slot blank. That way, the very first program you ran showed up on your Start menu immediately.
In Windows XP Service Pack 1, the assignment of the six slots changed slightly. Two of the slots were assigned by Windows, one by the United States Department of Justice, and the last three by the computer manufacturer. (Again, if you bought your copy of Windows XP at retail, then two of the computer manufacturer slots were assigned by Windows and the last was left blank.)