That KB article that Dana posted explains a few reasons why it's not as simple
as a option in Tools|options.

wrote:

"Dave Peterson" wrote:
since Lotus 123 thought that 1900 was a leap year,
that MS had a choice--do it correctly or do it to match
Lotus 123.
By matching Lotus 123's mistake, it would be easier
for users to migrate from 123 to Excel.
And since every company wants users and wants to
make that transition as easy as possible, the decision
was probably very easy to make.

As a developer of system software, I understand the
importance of compatibility. But assuming the explanation
is correct, there should be an option to correct the problem.
The default could remain bug-for-bug compatibility.

--

Dave Peterson