Harry seems to be wearing this sweater in a long scene during the movie. At first I thought this might be the sweater that Mrs. Weasley makes for him as a Christmas present, but then I realized that her sweater was "emerald green." So this may just be a random sweater that he has, though that doesn't seem to fit in with the story line, which specifies that the Dursleys only give him Dudley's old clothes, which are always 4 times too big for Harry. Since this sweater clearly fits him, and does not resemble a uniform of any kind, it remains a mystery where he got it and why.
Of course, it is always possible that the movie makers have changed Mrs. Weasley's gift sweater's color to scarlet to match his House. If this is Mrs. Weasley's sweater, then it might be fun to make! This is also a drop-shoulder crew neck, only this time it has cables running vertically. It looks like this sweater was also machine made (big surprise there), and it looks like pretty fine weight yarn. The cables are simple cables, probably 3x3 or something, with a few purl stitches to edge them, and the same amount of knit stitches between the cables. The cables also continue down the sleeves. There is ribbing at the waist and wrists, but the neckline is fairly loose/wide. This also would be easy to make, and is probably just as easy to find for sale at any store. Most people probably won't recognize it as being HP related, though, so it may not be worth the trouble. =)
Update 11/28/01: After watching the movie, it seems that this sweater was purchased by Harry with the money he got out of Gringott's. He also purchased 2 other sweaters EXACTLY like this one, one in blue and one in green. This red sweater is the one he wears at the end of the movie when he's saving the Sorcerer's Stone from Quirrel/You-know-who.