WIn98SE is still on the first choice. full compatible and fast!
WinME ... it's more stable but the i found it slower than Win98SE
Win2K.... nahh.. a lot of games won't run. it might run, but its very slow.
Win XP is a nice try... stable, powerful and so far i got no problems with it.
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u may want my option: Install WinXP, then dual boot with Win98SE (with
98lite add-on) 98lite is able to remove a lot of features from windows (name it: MSIE? media player? DirectX? old win311 apps?) and remove all things u dont wanna use. I install win98se just for compatibility.