A blue screen is followed by a failed auto boot-repair effort.
Which is the blue screen error code you see?. To have a chance of repair first thing you should do is search for its meaning and/or a known resolution about that specific error code on Google/Microsoft.
Is there a way to repair install the Windows 7 VirtualBox without
losing my non-OS data?
To recover your files instead of trying to repair the boot step failure, one thing that probably you could do is insert an ISO containing a Windows installation to perform a reinstallation of your OS in the virtual machine (ensure in BIOS that the CD-Rom boots before the HDD), this way your files from the corrupted installation will be kept in a new folder named "C:\Windows.old".
To do a clean installation and also preserve your current files in "C:\Windows.old", remember to select the "Custom" install mode: