How do I calculate the variance/covariance matrix (w/o using the data
analysis kit)?

Say I have a matrix X (each column of the matrix represents a
different variable), would the consistent variance/covariance matrix
be X'X/(N-1)?

However, when I calculate the covariance-matrix from the data analysis
program, I get a different answer (even considering the n/n-1
recalculation) from X'X/(N-1). They are close but still slightly
different. What am I doing wrong?

Thank you for your help.