Ooh - modthesims2.com makes it very easy now - if you download their Q-Xpress installer, you can just add things to your queue to download, and then once you tell it what folder to download to (which should be My Documents/EA Games/Ths Sims2/Downloads for everything except custom neighbourhoods) it'll unpack and install things for you automatically. I've also found good mods at thesimsresource.com and the official website thesims2.com

There's a whole Harry Potter Project, which someone left me a link to - if you look at my Draco/Pansy Sims post, it's in the comments there. ^_^