Thats not unity giving you an error, thats windows giving you an error, more specifically when I google it appears to be a firefox issue. Try a different browser and actually make sure you have the hard disk space free to download it.

Delete more. There is no way around the lack of space, hard drives can only store a finite amount of data and there is no way around that, and if your struggling to download setup.exe but I'm sorry, you won't be able to get any further. The actual install uses even more space than setup.exe, then your game projects will take a large amount of space. You need to get crazy removing stuff from your computer or get a bigger hard drive.

Do you have another hard drive? I for example have a 64GB SSD that has my windows 7 installation.... and not much else. I can't let my web browsers save files to the default locations, rather I choose a space on my much bigger 4TB(2 2TB in RAID0) drive. If that is your situation, than you need to click save as instead of just "save."

If that doesn't help any, I'd recommend a few things. Look in your programs in the start menu, and see how many you never use and can uninstall. You can also(and probably should) use the control panel applet to uninstall other programs that may not have an entry in the start menu. You can also go under the properties of your full drive and do a disk cleanup, and see just how much space gets freed. Also, check your recycle bin. Another option, that isn't as recommended, could be to remove your old restore points. Google that one if you don't know.