"Access to <name>.zip denied"

I have had this happen before, but cannot remember how I solved it.

I made a small track, put some Xpacks on there and saved it, now when I come to open Bobs Track Builder again I get the above message and an "Unhandled Exception" What do I do? the .zip is not showing any encryption, I have full control.

Push comes to shove I'm just going to buy Race Track Builder because I know Brendon has long given up on BTB. It's a good bit of software when it works though.