How to fix my acer extensa 4630z? Its boot manager is missing or corrupted.

First you need:
To enter Recovery Console in Windows XP, you will need to boot from the Windows XP CD.

Watch for a Press any key to boot from CD... message similar to the one shown above.
Press a key to force the computer to boot from the Windows CD. If you do not press a key, your PC will continue to boot to the Windows XP installation that's currently installed on your hard drive. If this happens, just restart and try to boot to the Windows XP CD again.

1. Enter Windows XP Recovery Console. The Recovery Console is an advanced diagnostic mode of Windows XP with special tools that will allow you to restore the boot.ini file.

When you reach the command line (detailed in Step 6 in the link above), type the following command and then press Enter.

bootcfg /rebuild

The bootcfg utility will scan your hard drives for any Windows XP installations and then display the results. Follow the remaining steps to add your Windows XP installation to the boot.ini file.

The first prompt asks Add installation to boot list? (Yes/No/All).

Type Y in response to this question and press Enter.

The next prompt asks you to Enter Load Identifier:.

This is the name of the operating system. For example, type Windows XP Professional or Windows XP Home Edition and press Enter.

The final prompt asks you to Enter OS Load options:.

Type /Fastdetect here and press Enter.

Take out the Windows XP CD, type exit and then press Enter to restart your PC.

Assuming that a missing or corrupt boot.ini file was your only issue, Windows XP should now start normally.