And, at 12.45am (having lost $190 at $5-$10 while trying to solve the problem), I finally got the machine running (although of course without the dual monitor). I had to boot up in "VGA" mode (one of the 8 or 9 options given to you when you press F8). This got the machine going, which was all I needed. I then stripped out anything to do with the SIS drivers. OK, the card is still in there, and it says "new hardware found" every time I start up, and I'll still have to get someone in to fix it so that it works with two monitors, but at least it's running. (Whoops, just got rivered again by JJ to beat my QQ. Make that $250 down)

But this really is an absolutely abysmal flaw in Windows XP, referred to in many many places on the Web, and no-one seeems to have known anything about it. Just type in "infinite loop" and "XP" to read of countless others who have experienced similar nightmares. Nearly always to do with graphics cards, Nvidia, and MRQ or something. Clearly the added video card caused the back-and-forth information between CPU and other hardware bits to keel over the machine. All the fixes mentioned online seemed to involve BIOS and stuff, but the starting in VGA mode seemed to do it for me. That's OK, I just couldn't stand the thought of it beating me.

Still wish that I hadn't installed the bugger, but at least now I can sleep without bursting into tears.

But what a wasted day, what a wasted, wasted day.