1. Confirm that Windows XP is not running in Safe Mode, as this prevents the Windows Installer Service from operating. If you aren't sure, click the 'Start' button and then click 'Turn Off Computer.' Click the 'Restart' button and allow Windows to restart normally.
2. Press the 'Windows' and 'R' keys simultaneously. Type 'msiexec.exe /unregister' without quotation marks in the 'Run' dialog box and press 'Enter.' The mouse cursor changes to an hourglass. After a moment, full control of the computer returns.

3. Open the 'Run' dialog box again. Type 'msiexec /regserver' in the box and press 'Enter.' The cursor changes to an hourglass again, then changes back to normal.
4. Attempt to install the program that is not working properly again. If the program installs successfully, stop here. If it does not, continue to the next step.
5. Open the 'Run' dialog box. Type 'regedit' in the box and press 'Enter.' The registry editor appears.
6. Click the 'HKEY_CLASSES_ROOT' folder on the left side of the window.
7. Click the 'Edit' drop-down menu at the top of the window, and then click 'Permissions.'
8. Click the 'Add' button. In the text box at the bottom of the window, type the word 'SYSTEM' and click 'OK.'
9. Click the word 'System' in the 'Permissions' window. Place a check in the box labeled 'Full Control' under the 'Allow' column, and then click 'OK.'
10. Close the registry editor and restart Windows. You should now be able to install the software.