Multi-tasking with defrag and system
You can use your system when the defrag is running. However, it is not recommended because the system is constantly changing files. The impact on overall performance varies with the configuration of your system.
File Access during Total Defense PC Tune-Up defrag
Yes, you might lose file access during Total Defense PC Tune-Ups defrag. Defragmentation creates an exclusive lock on files while it is moving them.
Defrag with security processes
Security processes like antivirus and firewall will not interrupt the defrag process. However, it is always recommended that everything be closed down during defragmentation. Applications sometimes create and delete files, including temporary files which can hinder the effectiveness of defragmentation.