Help! Monte Carlo simulation_ Importance sampling

[Destiny2000]

I try to price a call option very out of money using the importance sampling method in Monte Carlo. My results are not what I was expecting but I can't find out the problem_ It'd be grateful to have your help.
Here is the code in VBA:

Function MCOptionValue5(S0, K, r, T, sigma, c, nsim)

Dim rnmut, sum, sum1, S, F, W, W1, payoff1, payoffsqr, Z
Dim var1
Dim i As Long
Dim nRows As Integer
nRows = Application.WorksheetFunction.CountA(Range("B:B"))

Randomize

rnmut = (r + c - 0.5 * sigma ^ 2) * T
sum1 = 0
sum = 0
For i = 1 To nsim

W = Application.NormSInv(Rnd) * Sqr(T)
W1 = W - (c / sigma) * T
S = S0 * Exp(rnmut + W1 * sigma)
Z = Exp(0.5 * (c / sigma) ^ 2 * T + (c / sigma) * W1)

F = Exp(-r * T) * Application.Max((S - K), 0)

payoff1 = F / Z
payoffsqr = (F / Z) ^ 2

sum = sum + payoff1
sum1 = sum1 + payoffsqr
Next i
MCOptionValue5 = sum / nsim

var1 = sum1 / nsim - (sum / nsim) ^ 2

Cells(nRows + 14, 6) = var1

End Function

[bestdev]

A Monte Carlo method is a technique that involves using random numbers and probability to solve problems. The term Monte Carlo Method was coined by S. Ulam and Nicholas Metropolis in reference to games of chance, a popular attraction in Monte Carlo, Monaco.

Computer simulation has to do with using computer models to imitate real life or make predictions. When you create a model with a spreadsheet like Excel, you have a certain number of input parameters and a few equations that use those inputs to give you a set of outputs (or response variables). This type of model is usually deterministic, meaning that you get the same results no matter how many times you re-calculate.