I presume you mean windows XP? Isn't that 10 years old? Web builder has been removed from Unity 5. WebGL is now the only option in Unity 5 for building to a webpage. (As far as im aware.)
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I played hundreds Unity games on XP without problems (web and downloadable builds, exept the 0,01% around maded with Unity5). New OS are a downgrade of XP, are mutch heavy, slow, hardware eaters, full of random issues of all kinds, all sort of uncompatibilities, etc.. in other words, a full virus pack and nothing more. I've a full organized workstation with a lot of stuff: a OS switch will cause an enormous work to re-sync all (stupid Microsoft have changed all OS path names for example). New OS are business traps, I'll wait a serious real definitive one like ReacOS ;) http://reactos.org/