There is a keylogger you can buy at ThinkGeek.
Its called the "Key Katcher" and it connects to the keyboard, its a physical device that stores what he typed, so you'll only need a little bit of time to plug it in, and then again to get take it out, and read the information when you get to your own computer. The only problem is that he MIGHT notice it, and I'm not sure if it still works on Wireless Keyboards, but you can ask them
This is how think geek describes it.
This is a device that can be connected to a keyboard to record all
keystrokes. It has a changeable password, keyword search, enable/disable
option, and stores URLs. Records 262,000 keystrokes and does not require any software. Monitor unauthorized access to your computer or your network. Use it to troubleshoot or make fixes by tracing back through a users command sequence.
Key Katcher plugs in between your keyboard and your computer. A microcontroller interprets the data, and stores information in the non-volatile memory (which retains the information even when there is a loss of power.) This means that the Key Katcher device can be unplugged, and the information will not be lost.
To access the recorded data, you simply type your password in a text editor and the Key Katcher comes to life. A menu is displayed with options to erase data, view data, search data for keywords, change password, or disable the device.
Some neat features of this gadget:
* Available in the "Magnum" 4MB size, 256K, 128K or Mini size.
* It is Portable - move it from computer to computer.
* Installs in seconds - Just plug it in.
* Uses no system resources. Truly runs in the background.
* Works with all PC Operating Systems with PS/2 keyboards. USB version available here
* Data is retained even during system lock-ups and power outages.
* No Software to learn. Use in conjunction with programs you already know.
It can be installed quicker, but I would try to install a key logger program first, while shes away or something because programs are free and this is 50+ dollars.
I'm not sure if your willing to pay that much for it.
But, why not just confront her about it. Relationships are about trust and require communication. In my opinion, if you cant trust her then should you really be together? Think about it before you invade her privacy. Do you really trust her. You said you didn't want to do anything illegal. Illegal in the eye of the law, or illegal in her eyes? What if you keylog her and it turns out she isn't cheating on you? You SHOULD fill bad about that, and what if she wasn't cheating on you and she found out about your keylog. Before you invade her privacy, think about what your doing and its implications and consequences.
However, should you decide to spy on her...
1. Shut off spyware. (As was said before)
2. Install it in a file that is inconspicuous, and not hidden(that gives things away) make sure it is DEEP within the Windows system, where she would not find it unless she was looking REALLY HARD.
3. Make sure you don't open or view a document on her computer, it might show up in her "Recently used documents" and she might get suspicious. This means any document, but especially anything near the Key logger. There are ways to hide your tracks but it is easiest not to make them. And you probably wont have enough time.
4. Make sure it isn't shown in her "Programs list"(She is probably running windows)Some key loggers are cheap like that.
5. Try it out on your own computer so you know how to use it and have it all planned out before hand. Make sure you have a good one, and make sure you can put it on the computer with minimum activity/work. (See point 3.)
I would recommend one that remotely sends you the information, and if you can find one. You probably wont know when you'll get access to the computer again. However,You will have to get access to the computer eventually, in order to delete the key logger you installed. Or you can leave the program running, but remember, you run the risk of her finding it. (Realizing spy ware is off and turning it back on.) She might not suspect you, but then again, you never know.