Until System Rebooted Freezes Occured

I bought a new dell vostro about a month ago, pre-installed with windows vista home premium 32bit. All has been going well with it until Wednesday night when for some reason whilst using messenger the program froze, went light grey and a timer came up in place of the pointer.(i have a vague recollection of seeing a pop up regarding the power supply to the computer - not sure what it was) nothing else would work on the comp, ctrl/alt/del did not work so I couldn't force application to quit. Eventually just held power button until system rebooted. Since this first happened the computer continually freezes with no way of closing down programs, have tried a system restore, but even that process stalls and doesn't complete. Have tried uninstalling McAfee in case that was causing it but to no avail. I loaded up windows in safe mode which seemed to work fine, although the system restore was unavailable in safe mode. At the time it was downloading an 80mb vista update, which is yet to finish, again in safe mode the vista update was not running. The freezes occur with random programs, not just one particular one Anyone got any ideas of what the problem is and how to fix it, got a lot of uni work needing done outside of work hours and not having use of this computer is really annoying.