Hook, who at the time was still a semi-professional with Neath, celebrated that triumph over Ireland with the 300,000 other supporters who had flooded into Cardiff to mark the day.
It was a victory which inspired the greatest party Welsh rugby could remember for 27 years - and no shortage of hangovers - but for Hook it confirmed what he wanted from his life in rugby.
When Ireland return to Cardiff for Sunday's RBS 6 Nations opener, Hook will not be in The Yard, the Philharmonic or the Prince of Wales.
He will be running out at the Millennium Stadium alongside six of those Grand Slam heroes as a key figure at the heart of Wales' back division.
"When Wales won the Grand Slam in 2005, I was actually in one of those pubs across the road from the stadium," the 21-year-old recalled.
"Like everyone else, I couldn't get a ticket but it was still great to see Wales winning the Grand Slam.
"It is surreal the way things have changed so quickly for me. Watching the match that day just inspired me to become part of the Six Nations and I am thrilled to bits now.
"I had started playing junior rugby at the age of five so I had always been rugby mad. But when you see something like that and Wales winning the Grand Slam, it just spurs you on that little bit more."
Hook graduated through the Wales Sevens side, appeared at the Commonwealth Games in Melbourne and helped the Under-21s to the Grand Slam last season.
He made his Test debut for Wales on the 2006 summer tour to Argentina before he had even played for the Ospreys and then produced a stunning performance against Australia in November.
Comparisons with Wales legend Barry John and modern day New Zealand great Dan Carter soon followed. Everyone was getting carried away with Wales' latest discovery - everyone, that is, except Hook.
"It is quite surreal what has happened to me this season but I can't let it faze me, I just have to keep concentrating on my game," said Hook.
"Everything just seemed to go right for me in that game against Australia but not every game is going to be like that. I have to learn from my experiences."
Hook will reform his summer centre partnership with Jamie Robinson on Sunday after Tom Shanklin was injured and Hal Luscombe switched onto the wing after Gareth Thomas was suspended.
With Shanklin, Thomas, Shane Williams and Mark Jones all absent, Hook will have to shoulder greater responsibility than ever before - particularly as he is facing two of the world's best centres.
"It will be a step up for me. The Six Nations is always very intense, especially when you are up against centres like Gordon D'Arcy and Brian O'Driscoll. It is going to be competitive and I can't wait," he said.
"Those two are world class and because they play club and country together it gives them an extra bond which is priceless. But they have weaknesses in their game like everyone else and we will try to exploit those on Sunday."