Can't start external browser

Running Windows XP, using both IE 7.0 and Firefox 2.0 (default) browsers.
Using SL 1.19.1 (4), I start an internal browser (for example, hit F1 to see support page).
I hit the Open in My Web Browser button, and I get a Windows error dialog that references the URL and says:
"This file does not have a program associated with it for performing this action. ..."

Any idea what I might be doing wrong? Although the external browser doesn't start, the SL client seems unaware of the error.

When using IE or Firefox, I have no problems with local browser files, internet shortcuts, or other links, so the usual file associations should to be there.

My problem was caused by something not set properly in the Windows XP default browser settings. To fix it, I changed my default browser to IE, then back to Firefox.

On Settings | Control Panel | Add Remove Programs, select Set Program Access and Defaults to change default web browser.