Look on the software installation CD. If you see something like application_name.msi, (where application_name is the name or an abbreviation for the software, eg Photoshop LE). Try double-clicking it.

If it doesn't start installing then you may have a problem with Windows Installer. Open a folder, any folder. On the toolbar, click Tools>folder options. Click File Types tab. Scroll down the Extensions list to .msi. Make sure it says Windows Installer Package under File Types. If not, click Change button and scroll down to and select Windows Installer. Click ok back to desktop and reboot. Try the installation again.

If necessary, you can download the latest version of Windows Installer, free from Microsoft.

Ron

Thanks for that.

I have checked the installation disk and there is nothing listed that includes .msi. I have also been into Folder Options and Windows Installer Package is listed under File Types. It's worth repeating that this software has been installed on this computer before, and that suggests there is a problem with the computer configuration that has crept in more recently.

If I download the latest version of Windows Installer, do I first have to un-install the existing package, or just go for it?

David

Have you scanned for malware lately? 

Can you try a System Restore to a date when everything worked ok?

Start>help & support

Click "Undo changes..."

Follow the prompts and pick latest date available when Photoshop still worked. Personal files will be ok, but any software or Microsoft updates originally installed after the date used for the Restore will have to be reinstalled.

Ron 

You have solved the problem! By using system restore Photoshop has re-appeared. Thanks a lot!