VB preventing software to be cracked

Question

dear all

We have a construction company and I  am designing a program in Visual basics for it. This program is intended to be installed only on the computers in our office ( its not going to be distributed ).  There is always a risk that one of our employees will take our software and distribute it . Is there anyway to prevent this. If not how can we make this as difficult as possible??

any help or references would be most appreciated.