Select the range to highlight (I'm using A3:X999)
With row 3 containing the activecell:

Format|Conditional formatting
formula is:
=$H3<$G3
and give it a nice format.

The $'s will mean that columns H and G will always be used to determine the
formatting for that row. With no $ in front of the 3's, excel will adjust them
for each row in the selected range.

Coco wrote:

How do I tell Excel to highlight the entire row when data from two columns
does not match exactly, i.e. g3 should be exactly the same as h3 but if not,
highlight the entire 3rd row?

Thank you.

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Dave Peterson