On the note of Keyloggers, as I have worked as a computer P.I I can give you this advice.
1.) It must be your computer. You are allowed to monitor the actions taken upon your own computer. This is usually a more important question to ask with work computers or installing on employee laptops to ensure they are actually owned by the company and are not personal items of the employee.
2.) In many regions you have to state (to varying degrees) that users of the station may be recorded and state any limits on the uses of the machine.
3.) You may not like what you find. I got out of the scheme because I had to phone the police, turns out it was "barely legal" but still legal, but you might not be so lucky.
4.) As relationship advice, once the trust is dead the relationship is dead. Thats the whole point behind Othello. The only thing this will help is divorce proceedings to go in your favour.
5.) For software, at the time I would have reccomended "Elite Keylogger", I cannot guarantee anymore its still good.
6.) File recovery software is also useful, photo recovery software is the most common type.
7.) If you are in doubt what is legal, consult a lawyer to ask how to proceed in your region. Divorce Lawyers may be your best bet.