Simple excel math wrong
The problem is due to the basic hardware of almost all computers. They are based on binary systems so they don't count from 0 to 9, instead they just count 0 and 1, or base 2.
This is fine unless as you've discovered you want high precision decimal numbers. Computers can only count in terms of fractions that are powers of a half, i.e. 1/2, 1/4, 1/8, 1/16, 1/32 etc. There is no easy way for a computer to represent some numbers such as 1/3 or 4.99 perfectly accurately without giving up a lot of performance.
Excel is quite good at hiding this fact, it's only if you turn on 20 or so decimal places do you see this error. It does this by rounding, but only rounding when the result is displayed.
You say this is for financial calculations, do you really need the final result displayed to 20 decimal places? Or do you just need to be sure that the calculation is done with enough precision that rounding errors won't occur. If it's the latter you may just have to trust that excel has been doing it's job for a few years for countless financial organizations, without problem. And that you need not worry about those rounding errors causing you problems.
This has been answered on here before.
Just because you are using a "Round" number in base 10, doesn't help--the computer stores numbers in binary. Your round base 10 number translates to a number in binary that ends up repeating, you lose information when converting that decimal back.
Computer users get around this with two methods:
1) do your math to a certain number of decimal places then round them to less decimal places. 2) use BCD or some other exact decimal representation.
To the best of my knowledge Excel does not use #2, so pick another option.
there is an option in excel "precision as displayed" which effectivly auto-rounds numbers to the precision you choose to display your numbers at.
http://support.microsoft.com/kb/78113
would not recommend that you do that though as that may introduce rounding errors into your calculation as you are telling excel to round to 2 decimal places whereas currently the rounding error is to 15 (ish) decimal places so will not affect your calculations at all.