Hi, The PC has XP SP3 with all updates. It has a Pentium Allendale dual CPU. This BS comes up every time the PC boots, unless I choose "last known good configuration" (sometimes this option has to be selected several times before a successful boot): STOP: C0000221 Unknown Hard Error \SystemRoot\System32\ntdll.dll A bad MBR is detected when doing a FIXMBR. The boot after fixing the MBR is successful on the second boot attempt . FIXMBR then finds the MBR again bad the next time it's run. A minidump was created several weeks ago, but none since, despite multiple daily BSODs. A comparison of ntbtlog.txt files for successful and failed boots shows that good boots load ntkrnlpa.exe and then later intelppm.sys is loaded. Bad boots load ntoskrnl.exe and the first driver not loaded is ACPI Multiprocessor PC. If a restore point is created after a successful boot, the next time the PC is started it will usually BS with same error. Any help will be appreciated, Thanks!