Installing windows on late 2009 iMac 27"
This is what helped me. via http://ww3.tbreak.com/forums/topic/65304-boot-camp-repair-win7-partition-which-cant-boot-anymore/


  Posting this because it was a huge problem that has basically slowed
  down my work schedule by two days...
  
  Here's the scenario: You've created a new Boot Camp partition, or
  created a new NTFS partition on your hard disk using a third party app
  such as TUXERA NTFS-3G for use with your Boot Camp windows install.
  You restart your Mac and... BAM! No Windows anymore!
  
  What happened??
  
  Basically whatever normal operation you did before in creating a new
  partition has resulted in the MBR (Master Boot Record, which handles
  Windows booting) being corrupted. So how do you fix something related
  to PC when you're on a Mac?
  
  Before you start: First, make sure you can boot properly into Mac OS
  X. In Mac OS X, in System Preferences, select Startup Disks option.
  Choose your Mac OS X drive as the startup drive. You can press the
  restart button to test and see that the setting is saved. Second,
  uninstall rEFIt if you have it installed as it overrides Single User
  mode which is required for this fix. Third, if you have TUXERA NTFS-3G
  installed, disable it. Once you have disabled it, unmount and then
  remount your Boot Camp partition. This is so that the Boot Camp
  partition loads with Apple's native drivers rather than Tuxera's weird
  drivers (which are only useful within Mac OS X). Fourth, keep your
  Windows 7 install disc handy, you will need it. Don't look at your
  easy and convenient Windows 7 install USB flash drive, that won't work
  with a Mac.
  
  
  Shut down your Mac
  While holding down the "Command" and "S" keys, power on the Mac 
  Keep holding down these keys till you see a black DOS-like screen with
  white text 
  wait till you get a # mark allowing you to type commands
  type in fdisk -u /dev/rdisk0 and press enter 
  If asked to create a new
  MBR, press y 
  once the operation is complete type exit 
  Wait till it
  boots to Mac OS X 
  Insert your Windows 7 Installation DVD and restart
  your Mac 
  After the Mac booting chime, pres and hold the Option key 
  You
  should get options to select Mac OSX, another hard disk or the Windows
  DVD 
  Select the Windows DVD 
  Now you're in Windows land. Wait till it's
  done loading. 
  On the Setup screen where it shows options for language
  etc. press the Next button 
  In the second screen click the "Repair Your
  Computer" button 
  This brings you to a dialog where you are to select
  your windows install. If you don't see anything listed, don't worry.
  Select the radio button related to recovery tools and pres next. 
  You
  will get a window with links to several tools. Select "Startup Repair"
  If the operation completes and asks you to restart, click Finish 
  Keep
  the Windows 7 DVD in the drive, boot into Windows 7 setup again.
  Navigate to the Recovery Tools section again, This time click on the
  Command Prompt option.
  Type in bootrec.exe /fixmbr, hit Enter.
  Type in
  bootrec.exe /fixboot, hit Enter.
  Restart your Mac.
  
  
  Pulled this off after 13 hours of browsing through Terminal commands
  and Mac minutea. There goes the weekend!