Well I am not sure about that part, as I'm well aware of it, no one is.
The intro goes like this "Two millennium ago there was a war [...] Later, he quietly reigned the human world[...]. The legendary dark knight..Sparda."
I think that war can have lasted for centuries. I mean, Sparda could not fight everyone and immediately after that beat Mundus.(he would have been the scariest guy around hell). Mundus had an army. Sparda must have had some sort of army for himself is what I thought before, but I remembered that nature of demons and thought otherwise. Sparda must have fought his way through Mundus' headquarters and sealed him there. That could have taken years, seeing as then Mundus was at the peak of his power, and there was no gate between the 2 worlds so Demons were everywhere freely.
Also Sparda was not born with the knowledge of sealing the two words. He must have either been instructed by someone, or researched it himself. I also think he had been in the human world a couple of times before. Otherwise he wouldn't have liked humans, and wouldn't care about them. After all demons behave like they are brought up with the whole "humans are our food" thing, but the high demons - the bosses have established a way of thinking on their own.
Sparda probably did the same.
After all that war I think he must have defeated the most dangerous demons that lurked in the human world and THEN started the whole ritual sealing the gate thing.
Also by "reigned over the human world" I think he did pretty much what Dante does in his days. Hunting down demons. That explains why he had staff like "Luce&Ombra" which certainly are not a part from the Underworld.
By sealing "his powers" I don't think he gave up his demonic part. After all if he wanted to seal ALL his demonic part he should have sealed his whole self. I think he just sealed as much power was needed to seal the gate too. But in the end he IS a devil. I'm positive that he could still perform outstanding feats. And after all the devil blood ran through his veins forever. And through that blood Dante and Vergil got DT and all their other devil powers.
To sum up I don't think he lived 2000 years AFTER the whole sealing business. 2000 years in my opinion are the war between humans and devils along with Sparda defeating Mundus along with Sparda sealing the Gate and finally along with living some years with Eva.
It is very unclear, this whole 2000 years thing.
So all we can do is speculate^_^
