OK, here's the story:

My 360 and desktop PC are both on USB Wireless Adapters in my basement, and the wireless router is up in the attic. Now, my PC connection is fine (it actually seems to be faster on wireless than it was on wired), but the connection for my 360 is terribly slow. I constantly lag out of games, downloads are slow, I constantly have to rebuffer while watching Inside Xbox vids, etc. Now, I know it's not the router, because like I said, the PC is fast, any laptops that people bring to my house are fast, and even my Wii has fast downloads and what not; even though its (along with my PC) right next to my 360.

It's people who use wireless connections like you who ruin Xbox Live.
Throw away the wireless and get a wired connection.