Thanks so much. I followed the "help" instructions to enlarge my virtual hard disk (in Fusion, the Windows partition size automatically enlarges at the same time unless I specifically prevent it from doing so - at least for Windows 7).
Curiously, the alert I had been receiving had stopped popping up each time I logged in to my user account, but there was only about 1 GB free on my 40 GB virtual hard drive, and I'm sure my EHR installation, when it happens in a few months, will need a LOT more room than that, so I increased the total disk size to 60 GB.
Now I need to learn a bit more about snapshots, since those I cleared to enable enlarging the virtual drive were dramatically different in size among themselves.
Thanks again for your prompt answer.