Dell Inspirion_5720 questions?

For Christmas I got a dell inspirion_5720 computer. So far, I don't hate it, as it operates much better than my old computer (an Acer, naturally). I did, however, crop up a few questions that my untechnological parents cannot answer.

I have the option to upgrade to Windows 8 for a fair price of $15. Should I? Would it make my computer run worse, or better? What about the documents and downloads I have so far? Would they be erased?

Next, I have a Targus mouse that I used on my other computer. While at first it worked, now it doesn't do anything, and occasionally a window will crop up saying that it "doesn't recognize the device" or something like that. Is there any way to fix this?

Only one more question. My keyboard is automatically set to the US-International version. However, I prefer the plain old US version. Though I keep clicking US at the bottom of my screen, it keeps reverting back after a number of minutes. Is there any way to save this so it no longer happens?

While the answer to all these questions is helpful, if you only know the answer to one, do not hesitate to post. Thank you!