Win 2000 was my favourite OS for a long time, and I only moved to XP when it was a software requirement by Adobe apps and a few others.
Very robust with SP4.
XP wasn't really stable at all until SP2, but since then, when setup correctly was quite reliable, but I wouldn't say perfect.
The people having issues with XP were always a lack of ram, no free hdd space, or a rubbish graphics card.
For me I thought Vista was a big improvement over XP, but only if the hardware was powerful enough. On low-end hardware it could be unstable or slow, and that gave it a bad reputation. It was always hardware not up to the job, rather than the OS.
Then we finally reached Win7. For me this is the best OS to date. XP was a nightmare to install on certain setups. Not finding most hardware, so after install you would have a VGA low res screen, and most hardware with an ! next to it, and no way to ID it other than looking at the hardware.
Vista improved greatly on this, it would normally find most hardware drivers, but not quite perfect. Win 7 had refined this, and finds most hardware correctly. Only rarer things like BT drivers are needed.
And in use Win7, it is really stable. It never crashes, and handles unstable apps really well.
Again, anyone with bad experience of Win7 really needs to looks at the hardware, or any junk installed, as it isn't the OS.