How to update iOS with limited hard drive space? A friend of mine is trying to update her iPhone's OS through iTunes but it always fails as she has only 1GB free space left on partition C, and it seems that iTunes is using it as temporary space.

She's asking for help but I don't really know what I can do except using another computer with more free space to do the update.  Is there a way to download iOS manually so we can store it on another partition instead of using up all space on partition C?  Or is there any simpler way to do it?
A conventional spinning hard drive on a computer needs to have at least 15% free space for optimal operation. If a spinning hard drive has less than 15% free space, the entire computer system will run slower than it should, due to a phenomenon called disk fragmentation.

Your friend will do herself a favor if she buys a larger hard drive and pays a technician to install it and clone the contents of her older, smaller hard drive to her newer, larger hard drive. Then she will not only have a faster computer system, she will also have plenty of room for iTunes and the iOS upgrade.

Large hard drives are ridiculously cheap.

That being said, there are many things that can be done to clean out un-needed files from the existing hard drive to free up more space. Start by running the free CCleaner program.

Then free up space on the disk by deleting all but the last Windows System Restore Point. Do a Google search for instructions on that.