Thanks for your reply.
I also asked on answers.microsoft. Using advice received there, I found one computer had a driver causing the crash. It was an old one so I uninstalled the software containing that driver, as well as chasing around updating other drivers. It seems to be stable now.
The second computer did not respond to uninstalling that driver. The BSoD on that computer turned out to be caused by AVG antivirus. So I removed AVG and installed Microsoft Essentials.
The advice given me was to run Verifier, a Windows program, to stress the drivers. The minidump gave the information about what the problem was. It has taken a while, but we may be OK now.
Ref files locked by Dell Data Vault, how do I know there are any? I have not used the program.
Thanks again for your reply.