Yes, I'm using mine with my desktop. It even comes with a USB "extender" that moves the receiver out a couple of inches so you can plug it into the back of your PC and it will still pick up the signal from the keyboard.
I realized I didn't accurately answer your question ("can I use my wireless keyboard and mouse, and switch back and forth with this new keyboard with built in mouse please?"), and I wanted to compare this with a full-sized portable wireless, so I bought the Perixx PERIBOARD-716 Wireless Keyboard with Touchpad and plugged it in with this wireless model (the Logitech) already installed.
Yes, multiple wireless keyboards will operate simultaneously. I was running these two wireless keyboard/touchpad combos and my wired keyboard and wired mouse all at the same time.
However, I still prefer this Logitech Wireless Touch Keyboard K400 with Built-In Multi-Touch Touchpad to the larger Perixx because the Perixx would not allow me to enter my password when turning on my computer. The computer had to already be turned on, with Windows loaded and ready to go, before the keyboard would function properly. The Logitech does not have that limitation.
Help please....thanks for your offering the comments on "will it work with a desktop". Almost exactly what I need to know. Need to deepen a little if possible please..... I recently purchased a Dell Studio XPS desktop, and I have both wired and wireless keyboards and mouses (or is it mice?). Anyway, will this wireless keyboard with mouse pad conversion (or whatever it is properly termed or called) do everything that a normal computer setup will do please? And the main concern or question is - I know you mentioned switching between a wired keyboard and mouse above, but I like a wireless keyboard and mouse....... can I use my wireless keyboard and mouse, and switch back and forth with this new keyboard with built in mouse please? I am not even a tiny bit computer saaavy so any help or instructional comments will be greatly appreciated....
I'm able to type on the wireless keyboard and use the wired mouse at the same time; or type on the wired keyboard and use the wireless touchpad (mouse) at the same time; or type on one with my left hand and the other with my right hand at the same time. All three (wired keyboard, wired mouse, and wireless keyboard/touchpad combo) are working simultaneously.
As far as I can tell, this wireless keyboard/touchpad combo will do everything a wired keyboard/mouse setup will do. It's smaller than a conventional desktop keyboard, but if I had to I could discard the wired set and get by with just this wireless unit.
Regarding the password issue: Some reviewers have said that this is due to the num lock. You can turn it on/off to enter your password. With that issue aside, do you still prefer the Logitech over the Perixx? I am having trouble deciding between the two. I am looking to buy a wireless keyboard so I can use my TV as a monitor. What appeals to me about the Logitech is the additional left-click button and the sleek design, but I've heard that you can't scroll with the trackpad like you can on a laptop, and that the right Shift key being cut in half can be a big annoyance when typing.
Edit: I've also read that the touchpad works better on the Perixx.
Other than the password issue, I can't really say which is better because I returned the Perixx. However, I can say that the Logitech's pad eventually proved to be such a nuisance that I bought a wireless mouse so I wouldn't have to use the pad.
But I only use the wireless keyboard when I want to watch a show or movie from across the room, which accounts for about 10% of the time I use my computer. During the other 90%, I'm using the full-sized, wired keyboard that came with the computer.
If I was going to use a wireless keyboard only, and not just when I was across the room - and provided I could fix the password problem - I'd probably go with the Perixx. But for occasional use, I like the Logitech's compact size.
(Edited to correct typo: "If I was going to use a wired keyboard only..." to "If I was going to use a wireless keyboard only...")