iMac 27" shutting down during Windows 7 install
Get Temperature Monitor to see if it's heat related...

http://www.macupdate.com/info.php/id/12381/temperature-monitor

iStat Menus...

http://bjango.com/mac/istatmenus/

And/or iStat Pro...

http://www.islayer.com/apps/istatpro/

Open Console in Utilities &amp; see if there are any clues in the system log for Date &amp; Time of the last shutdown/crash &amp; the next start up.