Unfortunately, Symantec never offered a 10.00 to 10.03 update file, or free upgrade.
Symantec previously would offer a free upgrade to users of version 10.02 if you encountered a problem that was resolved with 10.03. The upgrade was never released as a patch file, it was a complete installation that involved uninstalling version 10.02 and reinstalling 10.03. Symantec would mail you a new installation CD, or you could download it from them. They don’t do this anymore.
If you only have 10.0 (or 10.01) , the only way to upgrade to 10.03 is to purchase a CD ROM from a third party, like on eBay (links to eBay WinFax search ) http://www.getfaxing.com/winfaxebay
then you could reinstall 10.03 overtop of 10.0 or 10.01
Is there a specific reason why you need version 10.03? Perhaps some of the problems you might be having can be resolved without 10.03.
Versions 10.0 and 10.01 are stable, even for Windows XP.