I have windows xp professional and when I run my

When you try to start a Windows XP-Based computer, you may receive an error message that the operating system is not found.
APPLIES TO
Microsoft Windows XP Home Edition
Microsoft Windows XP Professional
Symptoms
When you start a Windows XP-Based computer, you may receive one of the following error messages.
Error message 1
Operating system not found.
Error message 2
Missing Operating System
When you start your computer to the Recovery Console to recover functionality, you may receive an error message that resembles the following error message:
Setup did not find any hard drives installed on your computer.
CausesThis issue may occur if one or more of the following conditions are true:
The basic input/output system (BIOS) does not detect the hard disk.
The hard disk is damaged.
Sector 0 of the physical hard disk drive has an incorrect or malformed master boot record (MBR).

Note Some third-party programs or disk corruption can damage an MBR.
An incompatible partition is marked as Active.
A partition that contains the MBR is no longer active.
Resolution To resolve this issue, use one of the following methods, depending on your situation.

Contact your hardware manufacturer
Your best bet may be to contact the manufacturer of your computer or your hard disk. The manufacturer may have a utility that you can use to perform a more detailed scan for damaged areas of the disk and help verify the correct BIOS settings. However, be aware that the damage to your hard disk may be serious. Sometimes this means that your only solution is to replace your hard disk.

If a fix or workaround is not available, you can use the "Advanced Troubleshooting" section to try to resolve this issue.

Advanced troubleshooting
This section is intended for advanced computer users. If you are not comfortable with advanced troubleshooting, you might want to ask someone for help or contact support. For information about how to contact Microsoft support, visit the following Microsoft Web site:
http://support.microsoft.com/contactus/
Method 1: Verify the BIOS settings
Verify the computer's BIOS settings to make sure that BIOS lists and recognizes the hard disk. See the computer documentation or contact the hardware manufacturer for information about how to verify the BIOS settings.

After you verify that the computer's BIOS detects the hard disk, restart the computer, and then test to determine whether the issue is resolved. If the issue is not resolved, or if the computer's BIOS cannot detect the hard disk, you may have issues with your hardware. Contact the hardware manufacturer to inquire about how to resolve this issue. You may have to replace the hard disk.

For information about how to contact hardware manufacturers, click the appropriate article number in the following list to view the article in the Microsoft Knowledge Base:
65416 Hardware and software vendor contact information, A-K

60781 Hardware and software vendor contact information, L-P

60782 Hardware and software vendor contact information, Q-Z

Method 2: Use Recovery Console
Use the fixmbr command in the Windows XP Recovery Console to repair the MBR of the startup partition.

Warning This command can damage your partition table if a virus is present or if a hardware problem exists. If you use this command, you may create inaccessible partitions. We recommend that you run antivirus software before you use this command. We also recommend that you backup your data before you use this command. If the fixmbr command detects an invalid or non-standard partition table signature, the fixmbr command prompts you for permission before rewriting the MBR. The fixmbr command is supported only on x86-based computers.

For more information about Recovery Console, click the following article numbers to view the articles in the Microsoft Knowledge Base:
314058 Description of the Windows XP Recovery Console 307654 How to install and use the Recovery Console in Windows XP