Windows not logging

Hello,
i have bought a new laptop and connected it to same broadband network as my old one. now i am not able to start my older computer although both worked alright for initial 3-4 days. but now my old one on switching on shows that windows are logging on and then just crashes suggesting error i cant even read the screen as it happens so fast and keeps doing that same thing automatically i.e.trying to logon and crashing and loogon and crashing again and again until switched off again .