Well buddy, you can use the built-in WinXP Scheduler or some external util like GigAlarm to launch any specified program at a given time.
So u can go like this :
1.) Search for your favourite stuff and set the files for download in ur LimeWire (LW) client.
2.) Shut down LW.
3.) Now u can use the scheduler to launch LW at any given time. Since the filenames are already in the download list, the client will search and download then by itself.