The Glasgow Warriors flanker, who will win his 19th cap for Scotland, was part of the side that defeated the Pumas 26-14 in the second Test in 2008 and is hopeful of taking a 1-0 lead in the two-match series at the Estadio Monumental Jose Fierro in Tucuman this weekend.
Barclay said: "I think we have a similar spirit to the last time we beat Argentina. I consider that we are now a better team than the one which was here two years ago.
"In 2008 in the first Test we were very close (to winning], but we improved in the contact area and improved our shape and secured a good win in the second match."
Barclay also believes that Scotland have a stronger squad than two years ago, something shown by their much-improved Six Nations performances where they defeated Ireland in Dublin and should have beaten Wales, England and Italy.
"There are a lot of different players now who have the Six Nations behind them and have beaten Ireland," he said.
"We know how hard it is to face these kind of Test matches, but we also know it's worth all the effort when we clinch good results.
"We are very excited about these Tests. I think there are no reasons to think that we cannot go well in both games."
Argentina coach Santiago Phelan has announced the return of Felipe Contepomi, who missed all the Tests the Pumas played last year due to a knee injury.
Barclay is aware how important Toulon's Contepomi, who will play at fly-half tomorrow, will be for the South Americans, and is also wary of several other big names in the Pumas line-up.
He said: "I think Argentina have a powerful back row. (Juan Manuel] Leguizamon and (Juan Martin] Fernandez Lobbe are very strong players, Contepomi as well. Honestly, Contepomi can be one of the best fly-halves in the world. We have to watch out for these guys and try to tackle them."
Barclay is also looking forward to finishing coach Andy Robinson's first season in charge on a high. "Our level had ups and downs during this year," he said. "We had some very good wins against Australia and Ireland, but sometimes we have also played poorly.
"We are aware of the difficulty of playing Test match rugby and in order to beat Argentina we have to play at a consistently high level. Our mind is set on winning these two games, which are not friendlies or warm-up clashes. For us, they are very crucial as we are facing the end of the season."