I would say it's debatable about how similar they are (it greatly depends on your point of view).
Here are a handful of differences, between the two OSs( ignoring all but the latest versions of both ). I would name similarities, but I think everyone here agrees that the kernel( core coding/base code ) is the main thing that makes them similar(and nobody seems to have touched on the differences, which is the reason I'm posting here
).
Differences
1.WinXP has greatly increased Win9x and DOS compatibility
2.WinXP allows FAT, NTFS, and soon WinFS filesystems (WinFS should be public within a few months),Win2000 is only capable of creating NTFS partitions (to the best of my knowledge).
3.XP is capable of using 64-bit apps
4.XP has that weird cd-key/ownership validation( I'm not sure what the correct term is )
5.I understand that some Longhorn software/features will be "backported" to XP, such as WinFS filesystem, and not 2000( WinFS is the only one I can think of, off hand ).
7.WinXP has a more "user-friendly" interface(perty colours
)
8.Increased software support (and increased support from Microsoft, compared to 2000)
9.I think there is more support for multiple users using one machine in XP, but I'm not completely sure
Win98SE/Win95
Even Win98SE has major differences to Win98( stronger USB support, for one ), and the differences between 95 is greater.
Example=some programs, such as Hijack95, were quite buggy in Win98(almost unusable).
Feel free to correct any errors I have made in this post( in reference to the subject/topic at hand ).