"The Windows Installer Service could Not Be Accessed"error message when you install a program in Windows XP

To resolve this issue, follow these steps:

1. Log on to your computer as an administrator
2. Click Start, and then clickRun.
3. In the Open box, type cmd, and then click OK.
4. At the command prompt, typemsiexec.exe /unregister, and then press ENTER.
5. Type msiexec /regserver, and then press ENTER.
6. Verify that the SYSTEM account has full control access permissions to the
   HKEY_CLASSES_ROOT
   hive in the Windows registry. In some cases, you may also have to add Administrator accounts. To do so:Important This section, method, or task contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For added protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs.
7.  
   1. Click Start, click Run, type regedit in the Open box, and then click OK.
   2. Click the following registry hive:
      
      

      HKEY_CLASSES_ROOT
   3. On the Edit menu, click Permissions.
   4. If SYSTEM is not listed in the Group or user names list, click Add, make sure that the local computer name appears in the From this location box, type system in the Enter the object names to select box, click Check Names, and then click OK.
   5. Click SYSTEM in the Group or user names list, and then select the Full Control check box under Allow in the Permissions for SYSTEM box.
   6. Click Apply, click OK, and then quit Registry Editor.
8. Restart the computer. If the problem is not resolved, repair Windows XP

Courtesy : sysadmindop.blogspot.in &