Yahoo Finance has the first week and first month prices. According to Yahoo Finance, the first day of trading was March 13, 1986, which is one day earlier than the date that is on the eHow article I linked to. I think March 13 is correct (it matches the date on a file I found on Microsoft's website for calculating the value of shares purchased from IPO: http://www.microsoft.com/investor/Downloads/Stock%20Informat...)
According to the numbers on Yahoo Finance, the first week arithmetic return (relative to IPO price, not the opening trade price) was, at close on March 19: (28.25-21) / 21 = 34.5%, and the first month return was, at close on April 11: (28.75-21) / 21 = 36.9%
Here's a link to the Yahoo Finance page I mentioned (prices rounded to quarter of a dollar): http://finance.yahoo.com/q/hp?s=MSFT&a=02&b=13&c...
According to the numbers on Yahoo Finance, the first week arithmetic return (relative to IPO price, not the opening trade price) was, at close on March 19: (28.25-21) / 21 = 34.5%, and the first month return was, at close on April 11: (28.75-21) / 21 = 36.9%