What causes this error?

I recently booted up my moms laptop because she was having issues and it said
"BOOT MGR MISSING"
I had to compeltly rebuild the boot manager file through a recovery disk via command prompt.
What causes this error? Last night it was updating through windows update.
The OS is Windows Vista.
It is fixed now, but I'd like to know why to make sure it doesn't happen again.

marsmimar said:

The three most common reasons that I'm aware of (other Forum members may jump in with other reasons) are malware, bad sectors on the hard drive, and changing or deleting the system partition on the hard drive (usually the C: partition.)

Assuming nobody deliberately repartitioned the hard drive, I'd do some thorough antimalware scans. Since no antimalware product is 100% effective 100% of the time these free on-demand scanners are highly recommended:

Malwarebytes

ESET Online Scanner

Windows Defender Offline

If the scans come back clean I'd next run check disk. I'd want to run the full scan which will attempt to fix errors and recover bad sectors.

Disk Check

I'd also check for damaged or corrupt system files by running a system file checker scan from an elevated command prompt (option two.) If problems are found, run the scan 3 times and reboot the computer after each scan.

SFC /SCANNOW Command - System File Checker

NOTE: The check disk and SFC procedures should be the same for Vista as they are for Windows 7.

It is fixed now, it has messed up TWICE, and I've had to reinstall vista 3 times since we got the computer.
It may be the hard drive. I might go and buy her a ssd and surprise her with it.
This is her work laptop. She needs it working at full potential.

marsmimar said:

irbullet said:

It is fixed now, it has messed up TWICE, and I've had to reinstall vista 3 times since we got the computer.
It may be the hard drive. I might go and buy her a ssd and surprise her with it.
This is her work laptop. She needs it working at full potential.

Glad to hear you've got it fixed. For the benefit of others who may read this thread, what did you do to fix the computer? Another reinstall of Vista? Run check disk? Something else? I tend to agree with you that the hard drive may be failing. Especially considering the number of times you've done a reinstall. If you haven't already done so, I'd backup everything and then perhaps run a diagnostic check on that drive. The diagnostic check stresses the hard drive. If the drive is already failing the added stress could push it over the edge. Many people recommend SeaTools for Windows as it will work on any brand drive. Another tool is HDDScan. Both are free.

SeaTools | Seagate

HDDScan

I had to get the recovery disk and rebuild the boot mgr manually via command prompt
The tutorial is on another site. Don't know if I can post it or not here.