Hi Darren,
A couple different options you could look at:
1. BASIC: Checkbox per product interest (or product range):
When someone views a specific piece of content, web page, or explicitly submits interest you can update the specific product value to true
2. ADVANCED: Lead Score by product
Bit more complicated, but you could setup different scores for products and based on behaviours associated to specific product pages and content choose to increase the individuals product score.
Its a bit more work to set up scores per product, but its really interesting insight and powerful segmentation.