Windows Expplorer has stopped working

Got myself in a pickle here. We've been using CCleaner for years, under XP Pro and Vista Ultimate, both under x86 and x64 systems; no problems there. A couple of months ago, our network changed to Windows 7 Ultimate x64 (some Upgrades, some New Custom installations). One system, a server went in Overhaul at a US Facility, the other four computers of the network (1 x Desktop and 3 x Notebook) are linked by a

OK!

Ignoring already that the account is unfinished, I'm wondering what this has to do with CCleaner.

Just throwing it out there: try running Explorer file manager as a separate process from the Explorer desktop.