Hi nikbob,
After the operating system is installed, the drivers should be installed in correct order. Please use this link which shows the right order: http://tinyurl.com/8xvx6xy
If you have the resource disk, install the drivers in the correct order or use this link to install the drivers for the computer: http://tinyurl.com/cgcakw3
Click the link above, enter the service tag and select the operating system from the dropdown and follow the order in installing the drivers.
Note: Please use the above link in a different computer and make a copy of all the drivers in a CD/DVD/flash drive and save it, then install them in Dimension 8400.
After installing all the required drivers check if you are able use the internet.
Check and reply for further help.