Stuck in a boot loop

I restarted my 8-month old 15R a couple of days ago, but it got as far as the desktop,  then reloaded the desktop,  then reloaded the desktop... you get the picture.  It's stuck looping through the last few seconds of a boot.  Nothing holds still for long enough to tap click or otherwise start something up, and the only F-keys I seem to have at my disposal are F2 and F12 which if I reboot holding them give me various choices but not 'safe mode'.  I don't know whether this is a hardware or a software problem,  and I don't know how to get into the laptop for long enough to do some tests. There's a system test behind F12 which I ran and which came up fine,  but the system is still acting like a slow-mo strobe light.  Anyone got any suggestions?

Worst case I'll take out the hard drive and back up the data - it's been a while... ;0( and then reset it to factory status.  Anything short of that would be good...

Cheers in advance for any light on the matter!