Windows Installer Service could not be accessed (even though it's running)

These are the recommended steps to take to solve this issue.

1. Log on to your computer as an administrator.
2. Click Start, and then click Run.
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.
   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 the Registry Editor.
7. Restart the computer.

If the problem is not resolved, repair Windows XP.