It is annoying to say, that you would be part of this.
Suggest your client an alternative. There are quite a handful of programs that let's you create a environment where you would have to checkout any file, do the work and then later check in. The difference in two timestamps with the username would give you what you need.
You might be just crossing the lines of privacy, if you attempt to do whatever you say in the way you have projected it. And moreover such applications can be killed using Task Manager and would lead to inefficient time tracking.
How would you simply exclude the work done on personal office files (like his Resume , per say) with the ones that you may need to track.
In case, your client is adamant about a time tracking system, tell him that you will develop a new - customized time tracking system and have your own file formats.
Funny, isn't it?