Hmm. I checked in the device manager, and it said the Uno R3 was on COM18. When I tried uploading to the board before, it would just use the default COM3, and that is why it errored. I set it to COM18, and it uploaded just fine! Thanks
Haha now I need to install all the librarys and code on this computer. It is worth it though to have everything on one PC!
The Arduino IDE when first opened, just asks the PC's OS for a list of all active comm ports, the IDE has no idea which active comm ports might be attached to arduino boards or just standard comm ports available on that specific PC. It's up to the user of the IDE (you) to select the proper comm port number for the attached arduino board before doing a upload action, only you can tell it that information. The board selection requirement is so the IDE can tell the compiler what type of AVR controller chip your specific arduino board uses, and must also be made before an upload operation can be performed successfully.