You really should offer two different GUI versions to download, one for Windows 7 and one for Windows 8.
Example: (Or)
If you run CCleaner on Windows 7 it should ask if you would like to enable Windows 8 mode or keep the Windows 7 mode. (Mode means the new GUI).
I think that bunch of CCleaner users might stop using CCleaner if there's no any chance to revert back to Windows 7 GUI, sad but true.
Try to use different folder than Downloads, and try to pin it to Desktop (like Windows). I don't have a MAC unfortunately. Don't use drag/drop because I might cause some problems.
I meant that if you use older IE version for example it might not show slim version, but I'm not 100% sure though because I don't have XP.
You can always try this direct slim version download link:
http://www.piriform.com/ccleaner/download/slim
It should work on XP: Windows 8.1, 8, 7, Vista and XP. Including both 32-bit and 64-bit versions, but not RT tablet editions.
Try to use different web browser.
You can also create a .bat script to easily change between DNS addresses, here's a tutorial for Comodo Secure DNS:
http://tweakscene.com/viewtopic.php?f=167&t=7268
In my opinion you could try MVPS Hosts instead of DNS, I don't trust DNS providers because of this:
http://thehackernews.com/2014/09/google-public-dns-server-spoofed-for_16.html
Right click your desktop shortcut and select Properties, then select Compatibility tab, then select Run this program in compatibility mode for and select Windows 7 and then tick Run this program as an Administrator box and then hit Apply and OK.
This might help.