I'd offer two more suggestions.
1.
http://www.goozex.com - used game trading/selling/buying
2.
http://www.gog.com - Good Old Games - DRM-free downloads
With Goozex, I just did a quick search and found 1888 PC adventure titles listed. Now, I doubt that we'd call most of these actually adventure games, but still there's a pretty good bet that the ones you might be interested in will be listed. I got a copy of Divine Divinity through Goozex for $5, for example (not an adventure game, though).
Good Old Games is run by the folks that did The Witcher. You purchase a game and often extras (all downloads) that have no DRM (digital rights management) for a low price, and they all run on XP and Vista. I got a copy of the original Fallout from here, and it runs just fine. They are in a beta period right now, but you can still sign up, I'm pretty sure.