When is existing data destroyed?

Problem: My laptop with Acronis True Image 10 was stolen.
The thief tried to start it but the password to get into WindowsXP defeated him
He took it to a computer store claiming he had forgotten his password.
They started the computer and during the boot sequence started Acronis to restore the original XP system (which had been stored there since the computer was purchased)
They got as far as the WindowsXP product key. The thief did not have it and the store was not going to install an illegal version.
Everything stopped there. The police came in, recovered the laptop and returned it to me.
I have the product key but before proceeding, I want to know if my existing data has been destroyed already. There is a warning when Acronis recovery starts saying that existing data will be destroyed. But I want to know if/when it really happens and if there is a way to roll back the restore/recovery procedure?

Any help gratefully appreciated.

Cheers
David