When your computer starts, the "BIOS" attempts to find the primary hard drive's active partition to read the first sector for the "Master Boot Record", it uses that info to load the rest of the OS. For Windows NT4/2k/XP the MBR is pointed to the NTLDR (New Technology Loader). If the BIOS looked at the wrong drive or the wrong partition, if the partition wasn't active, if the MBR was damaged, or the MBR didn't list NTLDR in the right place, or the location of NTLDR changed, if the system was not yet fully setup, or if you had a hardware failure in one of the connections (cable, drive, motherboard), or if your hard drive got erased, you'd get the error. Why did your software configuration change? Often, after having uninstalled a program, but many possibilities exist, the good news is your data is likely safe, so let's keep going: http://ntldrismissing.com
Allright, check wikipedia's article about the boot sector: http://en.wikipedia.org/wiki/Boot_sector
Maybe I'll get to that some day. For now:
Hardware > BIOS > Boot Sector > MBR > NTLDR > Boot.ini > system32hal.dll > etc > Windows
Want it super technical? SeeWindows XP Resource Kit then click the "Troubleshooting the Starup Process", to get it straight from Microsoft.
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