Once I had my BB plugged into a computer just to charge it, and happened to run Windows Update, and it saw that I was missing the drivers and installed them!
I'm not familiar with the layout of the CD, but the drivers that are installed with Desktop Manager are installed under:
C:\Program Files\Common Files\Research In Motion
Simply pointing to the USB Drivers folder will stop Windows from complaining that it can't find the drivers when you plug up your BB. I'm not sure if the Serial drivers are simply for (very) old BB's that had only serial cradles, or if they're used for the serial Modem set up for tethering.