Installing ArcGIS 9.0 Pre-Release's 'ESRI_Library' Software Library CD removes user profile information. This data loss can range from loss of customized desktop settings to loss of mapped drives.
This is a known issue.
ESRI is currently working to correct the corruption and restore lost environments on systems where the ArcGIS 9.0 Pre-Release ESRI Software Library CD has been installed.
Do not reboot the computer. Future restoration may depend on a registry backup that can be overwritten on reboot.