Try a pgpwde --decrypt command from windows command prompt
such as:
cd c:\program files\PGP Corporation\PGP Desktop (or program files (x86) if you are on a 64-bit OS
pgpwde --decrypt --disk (disk number) -p "passphrase of user here"
If you are unsure what users are added to the disk do a:
pgpwde --list-users --disk (disk number)
If you aren't sure which disk is encrypted do:
pgpwde --enum
This should list which disks are online and encrypted
Let me know the results, if you cannot decrypt there. You might have to try a recovery CD or putting the drive in another machine that has PGP Desktop installed to decrypt.
While it isn't fully encyrpted, it sounds like the encryption process started and instrumented the drive but was then paused.
Good luck!
Ben