May be you are connecting the usb port and the dc source at the same time, in that case yo can have big currents between the computer and board. I've had similar problems with a usb oscilloscope because it's very difficult to isolate the usb bus so it's ussually referenced to ground.
I hoe it helps,
Once again, thanks to Jan and Raphael for their help for getting me started with a template for the C codes on SDCC fusebit syntax. My newest version of the LED blinking code is posted on http://www.dutchforce.com/~eforum/index.php?showtopic=14788.
However, I am having a problem of my PIC18F2620 not starting the control execution properly when I use my DC adapter as the power source. Eventually, I will be using the microcontroller on a portable battery, but for now, I'm using a DC adapter for development and testing purposes. I am suspecting that either I'm not setting the fusebits properly or I'm experiencing a problem with my power supply, which might require me to set up brown out reset and power up timer. How can I tell whether I have a fusebit configuration problem or whether I have a problem with my power supply?