Can an a XP computer be encrypted or decrypted while in Safe Mode?
I'm not sure if that was tested. I would recommend you to slave the disk in another machine with the same version of PGP Desktop and decrypt the attached disk.
Avoid the recovery disk (bootg.iso) because it will take long time and, the process cannot be stopped without risking data integrity.
Troubleshooting: PGP Whole Disk Encryption Recovery