Each time you press the "Power" button on your HP Presario 2100, it looks to a number of different devices to get the operating system files it needs to boot up. Once the computer finds the files it needs, it ignores the rest of the devices in its boot menu and continues to load the operating system. If you need to have your computer look first to the USB flash drive, you first need to enter the setup menu and change that boot order.
Log on to the Presario 2100 and click the "Start" button. Choose "Shut down" from the menu and confirm the shutdown at the next screen. Wait for the computer to shut down completely.
Press the "Power" button on the front of the computer. Wait until the screen goes blank, then press the "F10" key to enter the setup menu.
Press the right or left arrow key to highlight the "Advanced" menu. Arrow down to the "Boot Order" option.
Highlight "USB Devices" and press the up arrow until it is at the top of the boot order. Press the "F10" key to save the settings and exit the setup menu.