While the default drive letter F is designated in the WinPE PXE configuration, drive F is being assigned to another device during the WinPE boot process.
Another network drive was created (for storing images) using a share location also called "eXpress". The Altiris drive was being assigned to the wrong "eXpress" share location.
The password in the WinPE PXE configuration is invalid.
Edit the WinPE PXE configuration and move the drive designation for the eXpress share from F to another lower letter such as I. Rebuild the entire WinPE boot file and save before testing the new configuration.
Rename the alternative image location to something other than "eXpress".
Edit the WinPE PXE configuration and enter a correct user name (must be local admin on the Deployment Solution server) and password. To to the Tools pull-down menu > PXE Configuration > Choose the WinPE configuration > Edit> Edit Boot Image > Browse to credentials and enter correct information. Finish the Wizard and allow a new boot image to be created. Remember to click on the Save button on the main PXE Configuration screen.