did you do an upgrade to windows seven from a previous version of windows,or was it a clean installation?
and just to rule it out,did you install any antivirus/firewall afterwards?
although i haven't looked at seven's F8 menu yet,I'm pretty sure that it must have an option to stop windows from restarting itself due to system failure.so if you can boot with that option on you'll probably get to see a bsod with info about what's causing the failure