Screen receives no video signal after try-out on new computer

Hello,

I bought a new Acer Aspire XC-605 (INTEL CELERON G1840 2.8 GHz 2M, NVIDIA GeForce GT 705 1 GB, Linux), and connected the screen (Acer) of my old computer to it but got no video signal. I also tried with the screen (Philips) of my son's computer, but no signal either. The strange thing is: when reconnecting the screen to my old computer, it did not receive a video signal either. Only after fiddling with some cables (I think it was the keyboard connection), video signal came back. However, when re-connecting either screen to my son's computer, video signal still isn't back, even after disconnecting and reconnecting all cables, restarting computer in various ways, etc. It seems that the screen got "infected" by the new computer, and carried something to my son's computer that made it stop giving a video signal. It's not the cable connection, because disconnecting the cable is noticed by the screen (check cable connection), and it's not the screens, because both now work fine on my old computer. My son's computer worked fine before this, and it seems to start up normally (making the same sounds, etc.).
Addition after having been back to the store: my new computer had a VGA connector that was covered; I removed the cover and tried to connect with a VGA cable, but I shouldn't have, because the VGA port is "not active". Video via the DVI port works, and so does it on my son's computer. Tried system restore on his computer, without effect. My son's screen doesn't have a DVI port, so we'd have to buy a new screen for him if we can't connect it with a VGA cable.