Alright, this is pretty common actually. What video card did you have in before? Savage II or something right? Make sure you TOTALLY clean out the drivers for that. So, do this. If you are using Windows 98, fisrt uninstall any video utilities you have installed (using add/remove programs), then go to device manager and remove the video adapter and install the generic WINDOWS STANDARD VGA ADAPTER. Then reboot with the voodoo5 in. Windows should detect the card and either ask you for drivers, or try to install ones it has. If it asks for some, install the new ones, otherwise wait till it finds the old drivers then instsall the new ones. that SHOULD do it.
I just think your old video card drivers are screwing you up.
Good luck,