Training and certification tracking

I would like to track training certificates somehow.

Example: I have employees that take a training course, they are issued a certificate. Most of the training courses need to be re-certified after a year or two.

How hard would it be to create something in the HR module that would allow the certification info, certificate, date and renewal date, and remind via email when the certification expires?

Not hard at all. Just create a doctype with all the fields you want, then write a sever script that checks once a day to see which ones are due to expire.