Excel 2003

I am trying to use a name in a formula in accounts, which change every month
and the name which I want to look up depends on the month I am producing.

The formula is
=SUMIF('Lookup Data'!$E$2:$E$2422,$C7,mar05lum)

I would like to derive the mar05lum from the lookup of another table as
follows.

Jan-06 Jan06act jan06tb jan06lum jan06luy jan05lum jan05luy
Feb-06 Feb06act feb06tb feb06lum feb06luy feb05lum feb05luy
Mar-06 Mar06act mar06tb mar06lum mar06luy mar05lum mar05luy
Apr-06 Apr06act apr06tb apr06lum apr06luy apr05lum apr05luy

I have set up one cell to return the mar05lum on the basis of the date, but
I am not sure how I can then use that in the part of the formula, so that it
looks up the range mar05lum

I am not sure how to do this - do I need to use value(C5) or text (C5), or
something else.

Hopefully you will understand this.

Brian