Well - as the instigator of this thread, I have a quick update for you all.
My Asus Vista upgrade DVD arrived this morning.
I did a clean install, removing all other partitions.
I installed the new Asus drivers from the supplied CD.
Sat in work, when previously I couldn't get (or hold) a WiFi signal to a distant hotspot, it now works perfectly.
It is definitely NOT a hardware issue. Drivers alone have fixed my problem.
I will try at home, where I also had dropouts even in the same room as an AP, and in the next room to my Router - but I'm expecting miracles now...
I'm very happy. Vista looks good on the screen, and the one (major) issue I was having with the hardware has also been resolved.