"Unmountable boot volume" error means the OS cannot boot from the hdd either because the boot files are corrupter or the hdd is going bad or connector/cable problems. Reformatting the drive should be your last option, before doing that you should do a CHKDSK followed by an MBR fix.
1) Boot from your Windows 7 install dvd, choose startup recovery options, then get into command prompt. If you dont have an install dvd or it isnt able to get you into recovery, download a Windows 7 recovery disk from Download Windows 7 System Recovery Discs — The NeoSmart Files
and use that. Now, perform a scan on the drive letter of your choice by running "chkdsk x:" where x is your drive letter. The manual scan options are:
- /F Fixes errors on the disk.
- /V On FAT/FAT32: Displays the full path and name of every file on the disk. On NTFS: Displays cleanup messages if any.
- /R Locates bad sectors and recovers readable information (implies /F).
- /L:size NTFS only: Changes the log file size to the specified number of kilobytes. If size is not specified, displays current size.
- /X Forces the volume to dismount first if necessary. All opened handles to the volume would then be invalid (implies /F).
- /I NTFS only: Performs a less vigorous check of index entries.
- /C NTFS only: Skips checking of cycles within the folder structure.
- /B NTFS only: Re-evaluates bad clusters on the volume (implies /R)
"CHKDSK x: /F /R" would perform a full scan (including bad sectors) and attempt to fix them.
2) Follow up the CHKDSK with a boot sector fix. At the CMD prompt, enter "Bootrec.exe /FixMBR" (without the quotes), then hit Enter. After it is complete, reboot your computer.