ZBrush can’t be assigned to open any file types on any version of Windows or Mac. It’s never been able to do so. Files must be opened from within ZBrush; they can’t open ZBrush by double-clicking them.
As for the problem that you’re encountering with the launch, something has changed somehow. Doing a full reinstall may help. (Where after uninstalling ZBrush you also delete its folder and the Users\Public\Public Documents\ZBrushData folder so that you’re back to a fully clean slate.) However, if that doesn’t fix the issue the most likely possibility is that something in your hardware is starting to have issues. It could be the hard drive (either a mechanical problem or a matter of having sufficient defragmented free space) or possibly a RAM module going bad.
For that matter, it could still be a virus. Just because your computer isn’t connected to the internet doesn’t make it immune. If you transfer files to the computer by any method – even a CD or a USB memory stick – that media can get infected and transfer the virus to your system. Computers that are not connected to the internet in some ways are even more vulnerable since even if you do have antivirus software running (which you probably don’t) it isn’t getting updates and so falls out of touch with the newer threats.