'Not Responding' Windows can be caused by a lot of different things. Some times it is caused by Windows, sometimes it is the program itself.
The most common issue I see that in is a resource issue. Either you don't have enough RAM or a program is trying it all up. I generally don't have an issue with my programs 'Not Responding' unless I have a lot of things open at the same time. I am only running 1 gig of RAM, but with Vista.
This is also caused when a program crashes. This can cause Windows to become unresponsive or just the program that crashed.
I have seen Windows become unresponsive when the HDD is in use. Massive HDD activity will cause some programs to not respond. I see this a lot when I just right-click my music player and select Play with Windows Media Player. It has to read the 30 GB music files. Photoshop will cause my computer to hang when I open up a 500 MB psd because it is loading a lot of that to a scratch file.
This shouldn't happen to you often unless your HDD is dying, you have too little RAM, there is a program causing it, or your install of Windows is becoming corrupt (generally through virus/spyware/malware or HDD failure).
Macs do have this problem as well, I have seen it. I think that OSX is just as resource intensive as WinXP. I work at my school's helpdesk and see a lot of Macs come in with things not responding as well as received phone calls. Truthfully, I generally pass those on to people who have more experience. I am mainly a hardware, Windows, Linux, and networking support.
As to homeofmew's suggestion, that will only make your boot sequence faster, although not by much.
Defragging will reduce your HDD seek times, which will free up the HDD faster and release the resources. That is a good suggestion.
Monitor the programs that are starting when the computer is. Remove any non-essential program. If you don't need it why run it? If you hardly use the program, I would stop it from starting (this may cause that program to take longer to start, but to me it is worth it if I don't use the program).