Does anyone know of a good, simple-to-use calculator or spreadsheet that will let me figure a true (net) rate of return in my 401k investments?

What I'm looking for is a downloadable spreadsheet that will let me input starting values for each investment allocation and/or the beginning balance of the account, add in the actual weekly contribution amounts (incl. employer match) over a specified time frame (month, qtr, yr, from inception), input the current balance(s) for each investment selection, and have it calculate the true return %, in both absolute and annualized terms.

Alternatively, if anyone can come up with the proper equation, I could use that to figure it out by hand. Any help is greatly appreciated.

My quarterly reports show rates of return, but they aren't net of expenses and the required disclosure isn't always clear.