4 gigs of pain
We're rapidly reaching (or have reached?) the point where it's standard to have at least 4 gigabytes of RAM in desktop PCs. This presents an interesting dilemma, because most people run 32 bit operating systems; 32 bits doesn't allow more than 4GB of RAM to be addressed. The ideal alternative is to move to 64 bits; all modern CPUs support x86-64. Unfortunately, it requires a massive porting effort to get everything working on x86-64 (drivers from third party vendors are likely to be the biggest problem), so we're not quite there yet.
In the meantime, there's a useful feature called PAE which allows up to 64GB to be addressed by a 32 bit OS. I was surprised to see, however, that neither Windows XP or even Vista support it, although the server-based versions of Windows do!
The cynic in me wondered if this was a deliberate attempt by Microsoft to stop people from using the normal desktop version of Windows for running big servers, but this seemed a bit too much, even for them. But the Wikipedia article has the actual reason: "desktop versions of Windows (Windows XP, Windows Vista) limit physical address space to 4 GB for driver compatibility reasons".
So poor Microsoft appear to be stuck between a rock and a hard place. They cannot enable PAE, which is, in a sense, a backwards compatibility feature, because doing so would break driver backwards compatibility. This would appear to be an example of a situation where the Linux-style hatred of stable APIs wins over maintaining backwards compatibility. One part of the problem is that Microsoft relies on third-party vendors for drivers. They can't just update their platform and the drivers with it, because they don't have any control over them.