WinXP+SP2. Stable, fast, easy to use, more backwards-compatible with older games than Win2k (f'rex, I installed WC: Kilrathi Saga on this system which my 2k box flatly refused to even attempt...)
Overall, I like it just fine, MS corporate attitude aside. If THIS was the Windows that'd been released in 1995, I think history would've been a bit kinder to MS...! :-)
Would I switch to a different OS entirely...? Not sure: depends on the OS, depends on what's available for it. Currently my needs are: games (particularly first-person shooters and flight sims, and there's not a huge amount of the latter on consoles so don't tell me "just get a console for games!"), video capture and DVD authoring, some home accounting work, surfing the web, working on my web-page with a nice WYSIWYG editor, and writing.
While I could do 80% of that on the Mac, it's just not the platform for up-to-date games, and I don't like the closed architecture. Besides, if I wanted Linux, I'd buy Linux.
I could do about 30% of what I want on the new Amiga. Given that it's only 30% and the cost of getting an Amiga that would even come CLOSE to my current system specs is VERY high, I don't see myself making a switch. The A1 (and any other amiga-like systems) are currently "would be nice" but way, way out of reach price-wise.