I asked why at my first job interview. Stated reason was that as our end users use XP (standard at their company), we should use XP. Makes sense.
I guess the cost of mac vs pc explains why macs aren't more popular in regular companies.
As for Linux, well, I think it's just about it being easier to buy PCs with windows than with Linux preinstalled.
If there's one thing that's killing corporations it's this narrow focus on acquisition costs and a near total disregard for support costs. People moan about Apple being "too expensive" all the time, but the reality is most computers are too cheap.
I hope one of the major vendors develops some alternative to Windows. HP is shovelling billions into Microsoft's pockets with nothing to show for it.
I used a windows 2k3/2k8 machine every day for 3.5 years at the same company developing software, and never had a support issue with it. On the other hand, I solely use Macs at home (and, since leaving the aforementioned job to work on my own company), and have also never had a support issue.
However, my grandmother has recently gotten ahold of a vista laptop and a MacBook laptop. She has really shown a greater affinity to the Mac.
Also, we have an internally supported and externally available standard Linux environment (LinuxCOE) which engineers (like myself), who run multiple OSes in their cube, can use.
Speaking for myself only here, I would say that the alternatives are available but we can't really ignore what the market demands.
The cost of an upgrade is the daunting part IMO. We need to buy a bazillion CALs (licenses) if we want to switch to Windows 2008 server-side, for example.
2) If you work remotely, RDP is way better. I've even streamed video over RDP and forgot that I was streaming it from a remotely located computer.
3) If using SSDs Win7 will have better perf and longer lifetime.
4) Virtual folders.
But I would say that if your machines are completely locked down, for example all XP machines are not on the internet, just some closed internal network, then I think XP might be doable. Once you touch the internet though, all bets off.