Boot the Windows 7 DVD Repair console, click through to Recovery TOols list to Run Startup Repair up to 3 separate times with reboots: Startup Repair - Run 3 Separate Times
Go to what's left of the desktop, place your Install DVD in the tray to see if it will autostart to run a Repair Install
from desktop. If not, access Task Manager using Cntrl Alt Del, on first tab start New Task and type d:\setup
in the Run box to see if Repair install can be Run. Replace d:
with your DVD drive letter if different.
This behavior on legit copies can be due to drive letter slipping although it usually happens during reimaging. Check that your drive letter is correct in Registry using this method: http://www.sevenforums.com/installation-setup/51503-moving-Windows 7-partition-another-drive.html
If all fails, copy out your files using this method: Copy & Paste - in Windows Recovery Console
Run HD diagnostics/repair full CD scan: http://www.tacktech.com/display.cfm?ttid=287
Followed by Disk Check
if it finds bad sectors to repair, since chkdsk will have already been run if needed by Startup Repair. Sometimes you need the surface repair before chkdsk will repair file system.