You can try installing a different launcher app via the Android Market web interface.
- On your PC go to http://market.android.com and sign in with the same Google account as on your phone.
- Find any Home replacement app. Click on the "install" button to send the app directly to your phone (if you have several Android devices you will need to select the phone.)
- After a few minutes try unlocking your phone. It should ask you to select a launcher app, so pick the one you just installed, or the default one.