The reason you are getting that error is due to the previous year's Prep file from the previous year getting corrupted. Here is the solution that works best

1. Go to X:\Lacerte (X:\ is for the drive your database resides).

2. Then go to the previous year of Lacerte you are installing- XXtax folder (XX is for the year). Ex: if you are trying to install Lacerte 2011, you should be going to the Lacerte 2010 folder.

3. Go to the Option folder of the year Ex: OptionsXX (XX is for the year)

4. Find the Prep file Ex: Prep.XX (XX is for the year)

5. Rename the Prep file- Ex: From Prep.XX to Perp.XX.old Save the change

6. Try installing again.