Try deleting C:\Users\<username>\AppData\Local\Windows Live Writer\ResourceCache (This is the vista / windows 7 path, let me know if you have windows XP and I can figure out the path). Then reconfigure your blog inside Windows Live Writer by doing Blog | Edit Blog Settings... | Update Account Configuration
If that doesn't work send me your Windows Live Writer log file? Help | About | Show Log File
bturner@microsoft.com
-Brandon Turner [MSFT]