.rar is a compressed file format (like a zip file). It should be safe to delete it.
If you didn't create it or download it on purpose, then it's probably not anything important to you. It's certainly not a part of Windows or a program. You could uncompress it just to see what it is, but since it could be a virus, I probably wouldn't. (I don't know of any programs that use .rar for temp files, or would store temp files directly on the c drive.)
Just as a guess, "img" stands for image, images are often followed by numbers, and it's a compressed archive file. It's likely a large image that's been compressed, or since .rars can contain multiple files inside, a collection of images. But there's always the chance of a virus. You could scan it with an antivirus program or uncompress it in a safe environment (on Linux) if you really curious, but if you know it's not a file you created or downloaded, then I'd just delete it.
[edit]
BTW, if you do delete it, it will go to the recycle bin, so it's not permanently gone.
[edit2]
The term image can also refer to things other than pictures like CD and DVD images, operating system images, etc.
[edit3]
Some antivirus programs miss some viruses, so don't assume that it's safe unless you really trust your antivirus program. (I don't generally trust mine that much.)
[edit4]
After a Google search, it appears it may be a leftover file from some download. You don't need it, just delete it.