Did you remember to uninstall the current driver before adding the card? It realy shouldn't make a difference as both the onboard and the new card use the exact same driver, but anything is possible as can be seen by the problem you are having. If you didn't then Windows should have just ignored the onboard and installed/updated the driver for the new card from the existing driver install (or at least it has for me on the odd occasion when I enable the onboard on mine for an extra monitor connection).
I've tried installing the driver several times on Windows Update, on the CD and it won't fix it.
definitely doesn't help matters any. Others may have a different opinion but I am one of those that will not and will always recommend not to install any video card driver update from Windows Update. Things have got better since the XP days, but they can and do still cause problems for quite a few people.
now before going the complicated route of removing all the previous driver installs let's start with something easy. Have you re-installed or checked Windows Update (this is one of the few drivers I am OK with from there) the monitor driver? LG doesn't have a Windows 7 version but they do have a Vista one that supports both 32 bit and 64 bit, http://www.lg.com/uk/support/product...oduct-profile#
If that doesn't fix it then there are a few ways to get rid of all the previous driver installs to get it back to the default Windows driver.