Here's my understanding of the process.
The bootex.log file is created when chkdsk is run. The information contained
in the file is then recorded as an entry in Event Viewer. When Windows is
restarted after completing chkdsk, the bootex.log file is deleted. This occurs
regardless of the results of the scan.
If you're curious, run chkdsk /R from the Recovery Console which can be started
by booting from your XP installation CD. Once chkdsk is finished, copy
C:\bootex.log to another location such as C:\Windows. Restart the computer.
C:\bootex.log should no longer be present on the system. But if you open
C:\Windows\bootex.log it's contents should match the information you find in the
Winlogon entry in Event Viewer. That's because the contents of C:\bootex.log
were copied there before it was deleted.
For info on starting the XP Recovery Console, take a look here.
Courtesy of Kelly Theriot, MS-MVP
Recovery Console in Windows XP
http://www.kellys-korner-xp.com/win_xp_rec.htm
Good luck
Nepatsfan