It took the visitors a little while to get into their stride, but once they did, the Rebels racked up four first-half goals.
Williams fired Slough’s real chance wide of the far post after running onto a pass from Simon Dunn. He later hit the post from a tight angle, with Dunn’s follow-up blocked in the goalmouth.
But the Rebels then scored two quick-fire goals in the space of as any minutes, both from James Dobson.
First, the winger deftly chipped the ball over goalkeeper Hugo Sobte from inside the box, and then moments after the hosts had kicked off again, he ran on to a lofted pass over the Windsor central defenders and played the ball past Sobte with a first-time effort from the edge of the box.
On 35 minutes Nathan Smart added a third, the defender’s header from Dobson’s free kick too powerful for Sobte to keep out.
And two minutes later Williams got in on the act, receiving Guy Hollis’ knock-down with his back to goal before spinning and beating Sobte.
Windsor almost got on the score sheet early in the second half with Ashley Smith seeing a shot blocked, before former Rebel Paul Coyne hit the bar with an audacious attempted chip.
Later, Windsor left-back Haydon Clack put a free header from a corner wide of Jack Turner’s goal.
Substitute Gavin James, who replaced Sean Fraser at the break, looked dangerous and skewed one effort wide, had another fierce effort tipped over by Sobte and was later denied again by the keeper’s legs.
Williams got his second goal on 75 minutes, side footing home another Dobson delivery, this time from a corner – and completed his treble with three minutes remaining, playing the ball high into the net after taking a pass from James.
Slough, still unbeaten in pre-season, travel to Tonbridge Angels for their final friendly match on Saturday before the start of the League campaign on 12th August.
Slough Town Line up
Jack Turner, Josh Jackman, George Wells, Guy Hollis, Nathan Smart, Nathan Webb (Mark Nisbet 72), James Dobson, Simon Dunn, Manny Williams, Matthew Lench, Sean Fraser (c) (Gavin James 46). Substitutes not used: Warren Harris, Brad Wadkins.