Angel never was cursed with a soul, therefore the Whirlwind never broke up.  Darla never came back for the master, and the mob in Prague wasn't a problem for four master vampires so Dru wasn't injured.  In short, none of the vampires ever came to Sunnydale.

Buffy lives life as normal just without the Whirlwind's interference.  The master kills her, but Xander gives her CPR without Angel's assistance.  She never falls in love with Angel and some other big bad comes alone in s2 to try and open Acathla.

When her 18th birthday comes along, the council for whatever reason, (up to author) decide that Buffy the Vampire Slayer has to go.  To that end they capture the most dangerous vampire they know for her Cruciamentum, the Slayer of Slayers.

To increase Spike's danger factor, the watchers get him worked up with taunts of how they captured him at the currant slayer's behest so she could increase her rep by killing the slayers of slayers.

When he's finally let loose in the house to hunt Buffy he's killing mad.  However, it doesn't take him long to figure out that something is drastically wrong.  The badass slayer who supposedly wanted his head as her trophy seems more than a little weakened.  Spike who does like to talk to his food, strikes up a little conversation while he's stalking her.  Throughout the night they do a little fighting, a lot of chasing and a smattering of get to know you chit chat.  By dawn, Spike doesn't want to kill the slayer.  In fact they are just a little bit friendly.

If the Council becomes a Big Bad or is resolved here is up to the author.

Spike escapes the house and holes up for the day intending to head out that night to meet back up with his family.  However, they come looking for him instead, tracking their kidnapped childe to the hellmouth.

Because Angel was never cursed their dynamic is slightly different.  Spike loved Dru when she first turned him, but a hundred years of being second best to Angel and treated as a lackey by his family has made Spike slightly bitter.  He stands apart from them, but still travels with them because he doesn't really have any other place to go.

When the Whirlwind hits town, Angel gets a gander at Buffy and falls into obsession.  Now Spike is trapped between his tentative friendship with the slayer and his obligations to his family.

Buffy is having a hard time reconciling her duty with her feelings for Spike.  While dealing with feelings of betrayal, because Spike is part of the family of the vampire monster that is hunting her.

Unwilling to see his new friend destroyed in the same manner he's seen other young girls over the years, Spike comes to Buffy and offers a truce.

Together they take down the whirlwind.  Happy Spuffy.