Problem solved.
I'd forgotten that I'd installed a task manager enhancement tool - Prio, which gives some useful stuff like "sticky" priority changes for processes, highlighting (in red) any processes where the executable has changed, an extra tab with TCP/IP connections listed, and a few other handy bits,
Great tool, but, it seems, incompatible with running VMware Player with 64bit memory access.
It must've been longer than I thought since I'd run more than one VM at once, as installing that was a while back.
So if anyone finds a way to get them to run together, I'd be interested to know, but until then, Prio is staing off my system.
Shame, as it's rather handy.
Now off to their site to raise it as a bug with them 