Receiving "no Signal" Upon Turning On Desktop And Occasional Screen-freezes?

i recently got a new dual-monitor desktop built by a friend and it worked fine for the first week or so. a few days ago it started getting hectic whenever i tried to turn it on, in that it would frequently do one of two things:

1) the screens would both go black and post "no signal"

2) both screens would freeze in place and it would recognize no actions aside from turning the desktop off the "no signal" issue only appears when attempting to log in or right after turning the computer on, whereas the screen-freezing would happen at any given time, whether it be logging onto the computer, checking facebook, or playing diablo iii. it currently takes me 3-4 attempts to get it running properly each day. everything in the computer is new, so it shouldn't be a problem with bad cables connecting the monitors to the tower or the ports that the cables are connected to. i've tried moving the cables to different ports on the tower, and it doesn't really seem to be affecting the problem. nothing is out of place or unscrewed inside the tower, so it's probably a software issue. the same friend that built the computer suggested that i get the latest nvidia drivers since the most recent ones were available on the 14th, whereas my current nvidia drivers are slightly older. i tried downloading the most recent one, but then i received the screen-freezing problem. whenever i attempt to download anything video-related online, the comp simply freezes up again. i can play Internet videos and diablo iii fine, just not download anything significant off the internet. my friend tried working with the comp as well, but he's at his wit's end and suggests that we get outside help in diagnosing the problem, so he recommended this site.

computer specs are as follows:

windows 7 ultimate (service pack 1)
intel core i7 cpu
64-bit os
x2 gtx460 video cards
12.0 gb of ram