Try tapping F8 to enter
Advanced Boot Options.
Is there a Repair My Computer choice? If so, boot into
System Recovery Options to run
Startup Repair repeatedly no matter what it reports, at least 3 separate times.
If no Repair choice, reset your BIOS to factory defaults, enter BIOS setup to set HD to boot first, then use the one-time BIOS Boot Menu key to trigger DVD.
Where did you get Win7? If you burned DVD yourself, confirm ISO integrity or download another, burn DVD using IMgBurn at 4x speed, or write to flash stick using
Universal USB Installer with Win7 in dropdown, boot using one-time BIOS boot menu key under USB, Removable or HD's.
If Startup Repair fails to find the problem, try System Restore from the Options list.
Infection may be blocking repairs. Download, burn to CD or write to flash stick
Microsoft Standalone System Sweeper, boot it to run a Full scan, repeatedly if it finds infection, until it is completely clean.
Then run Startup Repair again 3 separate times no matter what it reports.
Next access a Command Line from Recovery options list, run
SFC -SCANNOW Run in Command Prompt at Boot. If it repairs System files, it may need to be run more than once then run Startup Repair again.
Next boot into free
Partition Wizard bootable CD. Make sure Win7 or it's 100mb System Reserved (preferred) partition are marked Active. Then click on HD to highlight it, from Disk tab run Rebuild MBR, Apply. If this fails on System Reserved partition, change Active flag to Win7 partition, click OK, then run Rebuild MBR again, Apply. If Win7 doesn't start and you marked or changed Active flag, try Startup Repairs again now.
You can copy out your files using
System Recovery Options using this method:
Copy & Paste - in Windows Recovery Console. Read my comment at the end of the comments section on how to do this from options accessed via F8. You can also use Paragon Rescue CD freeware to rescue files.
Then boot Win7 DVD, follow these steps to get a perfect reinstall:
Reinstalling Windows 7