Windows 2000 was the best version I've ever used. Back in the day, when XP was relatively new, if I bought a machine and it came with XP I would try and retro fit 2000 onto it. The 2000 installation would be noticeably faster for what appeared to be a similar set of features.

Currently checking the history of Windows 2000. Seems W2K was aimed at enterprises while ME at consumers. A bit less than 2 years after W2K's launch, WinXP came out and unified the consumer and enterprise line.

Then, almost 8 years after WinXP's launch, MS released Win7. There's a lot of improvements between the 2, but I wonder what were the major factors that made users upgrade to Win7. I only remember a few pain points in XP that were relieved in Win7: file/app search, connecting to the internet, system updates, and window management.

Personal reasons for upgrading to 7 in order of importance:

64-bit memory space support (crucial, considering enormous browser bloat after the FF 3.5 era)

TRIM support for SSDs (I switched to 7 when consumer SSDs started to become affordable)

DX11+ for gaming (DX10 was a wash in Vista, but by the time I switched to 7, there were some games that actually did look noticeably better with DX11)

Half-Open connection fix for torrenting.

But I'm a power user, so reasons for regular users may vary.

I upgraded to Windows 7 over XP for mainline x64 support and new window management feature (aero snap). Also, being able to sort system tray icons, as well as the taskbar were very welcome.

I upgraded from XP to 7. For me the main plus was stability - with XP you had to reinstall from an image when it got screwed up quite regularly.