Changed chipset settings, now have white screen

i recently upgraded my gateway multi media from 2 gb too 4 gb , when i did i started getting a dimms error when restarting the computer, but was able to bypass it and the computer would run fine and recognize the added memory. i called the place i purchased the memory from and they told me i need to go in and change the settings under shadowing and it would go away. I tried this last night, i wasn't able to find anything under shadowing so i changed 2 settings under chip set setting. I now have a white screen and not able to get back into the bios screen to change it back. do you know what I should do to try and get back into bios so I can change this back. I have already tried taking it down and bring it back up numerous time, once hitting esc repeatably , also f2 and f10 nothing seems to let me past the white screen. Please help

You rock! that was so easy and my computer is back up and going. thanks !!! I still have the dimms error , but i cannot find anything that pertains to it in the system. I probably don't know what exactly to look for on this system. Do you? Its a Gateway GM 5688E , triple core with 32 bit ps2... I have also just bought vista premium 64 bit too install so it will recognize 8gb of memory,, I have not downloaded it yet as it SCARES ME !! I already have vista premium 32bit will it work replacing it with vista premium 64bit? the computer is compatible with 64bit..