Precision T7600 cannot update BIOS

I have recently upgraded a Precision T7600 to Windows 10, but it is running so slowly it is impossible to use.

It is running BIOS A13 so I am trying to update it to A17, but I cannot get it to update:

1. Support Assist gives me the update, looks like it is doing something, but on reboot it is still A13

2. Booting from a DOS disk and running the update, gets as far as 'preparing update' then reboots but does not update the BIOS (note the Intel Virtualization has been disabled).

3. Running the executable (T7600A17.exe) from a windows prompt does nothing.

Further, I have tried booting into Safe Mode. This works correctly and seems to be running normally.

Booting into Safe Mode with Networking is also very slow and virtually impossible to use.

Can anyone tell me how to update the BIOS and if I should update the network drivers.

Thanks

Brian