From Macworld | Yahoo: Burn your DRMed tracks to CD now:

On Sept. 30, Yahoo will shut down the servers that are needed to reauthorize music purchased from its failed Unlimited Music Store if it is transferred to a new PC, Yahoo said in an e-mail to customers. Re-authorization is also needed if someone upgrades their PC's operating system.

The only workaround for customers wanting to listen to their music on a new or upgraded computer after this date is to burn the tracks to a CD and then reload them on a PC.
The EFF has warned for years about the consquences for consumers when vendors decide to withdraw their support for DRM-encoded music, McSherry wrote.