That was weird.
Computer did something weird and virus-y. I noticed that the net wasn't running. Not unusual in itself. Comcast often turns off at some point on Saturday morning. System maintenance or something. I rebooted to see if the modem was somehow unsync'd from the system. After the reboot, the system date was April 3, 2014, and all the stuff in the systray was gone. Comcast was on, but all the usual American websites were showing as "uncertified". UK websites were okay.
4/3 was before I bought this computer, and before its software was installed. When I bought it in August, the date of the system files was 5/14/2014. So 4/3 shouldn't be a 'default date' for the machine.
Ran a System Restore to the last probably-good restore point, which seemed to straighten things out, but the date was still 4/3. Manually changed the date to 10/25, and it seems to be sticking there so far.
Ran AVG to see if any viruses showed. Nothing. But AVG had lost its paid registration and thought it was the Free edition. I don't know how long that had been happening; if it had been running as Free for a while, it could have missed something. I know from earlier experience that the Free edition isn't strong enough.
I wasn't doing anything unusual this morning; the usual web locations, the usual graphics stuff. Amapi, UVmapper, Poser.
Google shows April 3, 2014 as the date when the SSL Heartbleed thing was discovered, which is probably significant in Hackerland. But Google doesn't find any specific viruses that actually reset your date to 4/3.