The files are sorted in a way, that for exsample Firefox program files are near each other, so Firefox launches much faster.
Basically sums it up nicely, but there are things that all people should realize:
-If you have enough free RAM (for example, if you have 100MB of RAM used on XP out of 1GB of memory), data that has just been accessed by the disk will be placed into memory. This is placed into something called the page cache
which is transparent, it doesn't interfere with memory that applications or Windows can use, because the page cache is cleared to allocate memory for them, data in the page cache doesn't show up on the default Task Manager memory reading. After a couple of hours of usage, the average user will have "unused" memory being completely utilized.
When you reboot, all memory is cleared and hence the page cache. So, for example, try this:
Reboot your system.
Start Firefox (or whatever web browser you use).
Start Firefox and notice it launches much faster because it is in memory now.
Internet Explorer modules are loaded in memory on boot-up, so that's why it usually launches fast.
Windows Vista and Windows 7 have a feature named Superfetch which monitors data access habits and will automatically place data into the page cache without the user having to first read them with the disk.