Hi 
Have you tried using a specialist tool such as;
GnuCash, runs on Windows too
http://www.gnucash.org/
This one is reasonably famous. Competes with mainstream programs such
as Sage, Quickbooks etc.
Grisbi
http://www.grisbi.org/
(website down for maintenance right now and i've no idea how long it's
been). Never heard of this one before but that doesn't mean much.
Not sure if it runs on Windows.
HomeBank
http://homebank.free.fr/
Reasonably famous but i think its more for small business and personal
finance where you might be thinking more about larger scale. I got
their English page. Not sure if they have other languages too but i
suspect the original is in French. Does work on Windows too.
Skrooge
http://skrooge.org/
Again, never heard of this one but again that doesn't mean much. I
don't like the look of it because it's KDE-based and wants to install
tons of dependencies on my very default Ubuntu. Also they say
"possibly works on Windows".
So, without actually having tried any of them i'd probably go for
GnuCash or maybe HomeBank. I did open GnuCash once but it reminded me
of work so i quickly closed it again.
Assuming you have looked at those sorts of things and decided that
making your own might be simpler then in the future it might be easier
to get your existing invoice form to get it's data from a spreadsheet
rather than using this way around. It might be possible to pull in
data from separate forms into a single spreadsheet but it might be
easier to just copy&paste it this time.
How many invoices have you already done? If it's just 2 or 3 then
it's no contest. Just copy&paste. More than 10 to 30 would need some
thought. I'd be leaning towards automating or hiring a touch-typist
for a couple hours.
Regards from
Tom 