Uninstall fails

[havezet]

Some of my clients have reported that they can not uninstall my product or do a full install of a new version. The message they are getting is

Unable to uninstall old version of <<title>>. Please uninstall it yourself using Control Panel Add-Remove Programs before attempting to install this product. The installation source for this product is not available. Verify that the source exists and that you can access it.

I asked the customers to try MsiZap which worked for 1 customers but not for many others.

Does anyone have an idea how I can help these customers or prevent this in the future?

In case it matters, I use InstallAware 7.0 (build lonepine_rtm.071107)

Thank you

Hert