During the execution of a task sequence, you can monitor the progress in real time by looking at the report titled “History – Specific task sequence advertisements run on a specific computer”.
Note: Replace SCCMSERVER for the name of your server with the reporting role.
http://SCCMSERVER/SMSReporting_001/Reports.asp?ReportID=143
You can also reach this report by starting with the report titled “Status summary of a specific task sequence advertisement” and then drilling down.
http://SCCMSERVER/SMSReporting_001/Reports.asp?ReportID=145
In addition to the web reports the build process also creates log files locally on the computer. These are excellent for detailed troubleshooting. They start out in a “_SMSTaskSequence” folder in the root of the largest partition, but end up in the standard ConfigMgr client logs folder (C:WindowsSystem32CCMLogs on a 32-bit OS and C:WindowsSysWOW64CCMLogs on a 64-bit OS). The main log file is smsts.log, but there is also BDD.log and various ZTI*.log.