obviously trying to write to a temporary directory on C: to get the installer started.

Here's a useful thing to remember. Many demos packaged as .exe file are actually self-extracting archives. If you have the relevant archive software installed you can often manually extract the files to any place you like. I highly recommend having 7-zip(.org) installed as it handles 7z, zip, bz2, rar, and many other formats, and it generally produces the best compression ratios and so is a prime candidate for making self-extracting archives with.

With 7-zip I just right-click on the .exe file, and select the relevant "extract to..." option, and it will either do it, or give me an error if it's not a self-extracting archive.

Of course, some installers end up creating about three copies of the data along the way, so another trick I've utilised at times of low disc space is to watch the windows\temp directory, and take a copy of the installation files that are copied there, then copy those to a drive with more space, cancel the installation, and run it again from the copied files on the bigger drive.