Matrox emailed me to say that the triplehead2go is not set up for Win 7 and that they have no plans to make it so. Bummer..
I upgraded to Win 7 64 bit anyway, and sure enough, the disc that came with my (analog) triplehead2go didn't work. So I went onto Matrox's site and downloaded what looked like a sensible driver. Not sure what or where - go check it out. Maybe it was Vista 64 bit?
Anyway, it worked. I have three screens running fine on Win 7 64 bit off my analog triplehead2go. Very pleased, and quite surprised.
(screens are two 19 inch HP 1906s, and one Acer223W 22 inch widescreen - orientation 3840 x 1024)
Nb googling along the way, I came across a tip which I incorporated into my final successful setup. I think it was this: right click on your powerdesk icon, select Compatibility, and select, (ticking the box) Windows Vista (Service Pack 2). That's what I have set up now.
So... analog triplehead2go users, take heart, despite what those lazy bleeps at Matrox say, it is definitely possible to get three triplehead2go screens working on Win 7 64 bit with a sensible download from the Matrox site (don't ask me, I can't remember and I didn't save it, but I think it may be as I said it was) and that simple tweak.