ust a quick thoughs:1) The file that is being downloaded may try to be saved outside of W3 folder. So giving the program all permisions to W3 folder may not be enough. But running it as admin should solve it. So I guess this is dead end.
2) I'm sorry, but I'll have to try (from personal experience): Do you have a free space on your HDD?
3) I've recently encountered a similar problem at my work: Not having enough space on hard drive. I found out that my temp folder was set to other partition of the HDD.Check it (sorry, I don't have my Windows in English, but you should be able to find it):Right click on Computer, Properties, Advanced system settings (at the left side), Advanced tab, Enviromental Variables and here make sure your TEMP and TMP are on a HDD with enough space.
4) If you can, install W3 to different HDD or different partition.