Quick Coding Quandry - can I code this into a Gadget for HTML site?

Can you embed a clever, excel like gadget in simply HTML page?

A friend has very kindly designed and produced (in HTML) our website - http://www.napa-technology.co.uk

I want to create a gadget to calculate pay back etc - just like the one I've modelled in the excel document linked to in the title.

Just want to assess the feasibility as I'll be looking for someone to do this and want to know what i'm asking for!

Any comments on our new site are much appreciated - I like the design personally but think it could be modernised a bit in terms of layout, coding etc.