The encryption is based in part on your account name, which has a unique identifier that is set when the account is created. When you reinstalled your operating system, the user ID's were recreated and are different, even if the username is the same. The only way to decrypt the data is via a designated recovery agent (which was hopefully set by when you set up the folder). On a local machine, which is not part of a domain, the local administrator account is the default recovery agent and may be able to recover the data. However, if the administrator account changed during the reinstall, this won't work either and the data is unrecoverable.
This was first published in September 2004