microsoft setup vista so it would have a hard time running any hardware that did not have a digitally signed driver. in doing this they caused most systems at the time to have problems with vista. luckily they fix this with sp1. the problem was microsoft wanted digitally signed drivers form the hardware manufactures, but chose to bring the manufactures into the loop after they released vista.
That was for 64 bit only and was the same for XP-64 and the 64 bit server software. It's one of thise "nice ideas in theory" but yes, all it really does is make it even harder to actually move to 64 bit.
32 bit has never required signed drivers. The main problem were the vast changes in the audio and video driver models that a lot of hardware vendors were some what loath to have to keep up with, and in some cases very slow about it. THere were a LOT of smaller tweaks that required minimal effort to fix from HW/SW people but most o fthem didn;t bother to even look at it till after vista was released.
MS worked with manufacturers for at least 18 months BEFORE the release of Vista but many of them still chose to wait till Vista was released to really do much work. Then suddenly it was all a-panic.
You could really see the dividing lines between those companies that were on the ball and ready to take advantage of the new release (because/they/ wanted to sell new hardware) and those that really didn't seem to care at all. One such duality was MOTU vs M-Audio. MOTU had crack drivers even during beta, M-Audio took MONTHS to get drivers out for the bulk of their hardware (they are finally almost fully complient... finally). Then there was Soundblaster whining up a storm instead of coding

They were the worst, just whining and whining about how it was too hard and blah blah, while smaller companies jumped on it and got the job done.
So the moral of the story is: Never attribute to malice that which can be explained by laziness.