If when plugging in your BlackBerry Pearl 8100 you get the following message:
USB charging current is not sufficient. Verify that your handheld is connected to a powered USB charging source and that the proper USB driver is installed.
Then you'll need to install some software. In this particular case the error message actually makes sense, as you need to install the BlackBerry USB and Modem Drivers.
Get the drivers (v4.3) here: http://na.blackberry.com/eng/services/desktop/req_4dot3.jsp