You should probably buy some MSFT at the open tomorrow and fund it with a short position in AMZN. For posterity's sake, the closing price of both companies today was:
8/8/2013
AMZN: $295.74 @ $135.12 billion market cap
MSFT: $32.89 @ $273.97 billion market cap
8/8/2013 AMZN: $295.74 @ $135.12 billion market cap MSFT: $32.89 @ $273.97 billion market cap