NBU_VSP_Cache

I am new to using Netbackup and still trying to figure it out. I have Netbackup 6.x and running one Netbackup server with Netbackup clients installed on the servers I am backing up. These are all Win2k3 servers. There is a folder created on my servers on the C:\ drive called NBU_VSP_Cache that is growing rather large and when I try to move it to a different location I get an error that another person or process is using this folder and it want let me move it off C:\.

A couple of questions what is this folder used for? Image files?

And what process would be using it because I have tried stopping the Netbackup client service and moving it but I still get the message that some process is using this folder?

Thanks for any help,
Bill