It sounds like you're looking to do a basic mail merge? If the process is going to be user initiated then the steps to perform a merge are located can be found by googling mail merge.
I have't dug up a routine that does the entire process for you, but hopefully I can get you going down the right track. I would imagine that your ideal solution would be similar to the following:
- Programatically Create mail merge via excel/access
- Schedule intervals for the program to run
Assuming you're in a hacking mood then if you would like to automate most of the process here is a snippet for Access VBA, and two for Excel VBA here and here.
If you would like to automate the entire process (ie: scheduling, etc), then it may be a bit more involved. You can look at running a scheduled job to execute the process from windows or possibly look into using powershell to accomplish all of the above.
Hopefully this will start you off. Let me know if you're stuck and I'm sure we can cobble something together.