Sobana,
The explanatory variables (factors) you're considering are:
Group: A and B
Task: 1 and 2
Time: pre and post
Day: Day1 and Day2
To perform a full factorial ANOVA, each subject would need to have 8 (Task X Time X Day) input files. However, I believe that you only have 4 input files for each subject; that is, you have missing information about Task 1 for Group A in Day1 and Task 2 for Group B in Day2. In other words, factor Day is correlated with Task, and you won't be able to have both Task and Day in the same model.
Gang