Boot the Windows 7 DVD Repair console or Repair CD, accept any offered Repair. If it fails to start, boot back in, click through to Startup Repair to run up to 3 separate times with reboots to repair/rewrite the MBR if necessary.
If this fails, boot back in to try System Restore specifically although it should run as one of many fixes tried by Startup Repair.
Safe Mode should not load your ATI driver
but should default to the Standard VGA driver used during install. If this continues, open a Command Line from DVD or Repair CD, type: cd /d c:\windows\system32\drivers\
(if c: is the Windows 7 drive) del atipcie.sys shutdown -r -t 0