So, I thought I was pure class and elected to edit some stuff about the way my WIndows 7 looks. The log-on background was first. No trouble there, quite successful. Next came the picture frame for the user image on the same screen, which is obviously what's caused my black screen, as I used Resource Hacker and edited the authui.dll file. Of course, I made copies of that file first. I edited both the syswow64 and system 32 folders' authui.dll files.
I restarted to see my changes. It took for ever. There was a black screen with the cursor for a few minutes, and then it restarted. It brought me, as no-one is surprised to learn, to that same black screen with the cursor. No cause for the panic now, as I could simply reboot and use my Hiren's rescue disc to access that HDD and set things right. But here's where they get you: now it won't POST. My monitor displays its manufacturer name on the screen, so it works, but *maybe* the signal isn't getting to it? Pretty odd for both of these problems to happen simultaneously. Thanks for the help.