I have a mid-2011 Macbook Air running both OSX Lion and Windows 7 using Boot Camp.

Over the weekend I upgraded it to Windows 8 by doing the download only installation. The process was quite simple and it worked fine. Here’s the steps:

1. Boot into Windows 7
2. Go to http://windows.microsoft.com/en-GB/windows/download-shop and click “Buy Windows 8”. Download the installer and start it up.
3. Pay! The installer will ask for you billing details, credit card / debit card / Paypal details etc. At £25 I thought it was a cheap enough upgrade, especially since I would be going from Windows 7 Home Premium to Windows 8 Pro which means I’d get Remote Desktop etc.
4. Wait half an hour while the installer downloads the OS.
5. Once the install starts, it will ask you if you want to
   1. Keep all your programs and your personal info and settings
   2. Keep your personal settings only
   3. Keep nothing. I went for this option as I always like to start fresh.
6. Now I had a few problems. The install demanded that I have 20Gb free on my C: and I only had about 5 Gb free. I had to stop the installation to free up some disk space. I did this by:
   1. Running Disk Cleanup
   2. Disabled Virtual memory, which shrunk the pagefile.sys from 4Gb to 0Gb.
   3. Disabled hibernate, which removed the 4Gb hiberfile.sys
   4. Deleted c:\Windows\TrustedInstaller folder – but that was a mistake, because then I couldn’t uninstall any programs to free up the necessary space!
   5. Since I was choosing to keep nothing on C: anyway I deleted folders willy-nilly from C:\Program Files and C:\Program Files (x86)
7. After that I was able to continue the installation which took another 20 mins or so. It rebooted 3 or 4 times, during which  I had to hold down the Mac’s option key at boot and choose the Windows partition.
8. All done! Windows 8 works fine.

Once it was installed I did a couple of clean-up tasks.

1. My C: was almost full again, because the install had backed up everything in the C:\Windows.old folder. So I ran Disk Cleanup again and clicked the “Clean up system files” button to remove those.
2. I no longer had Apple’s Boot Camp drivers installed in Windows, so I had boot into OSX, start the Boot Camp helper and copied the Boot Camp drivers to a USB memory stick. Then rebooted into Windows and installed those drivers. Worth doing so that the Mac’s hotkeys (volume up/down, screen brightness etc) would work.