Microsoft released a Visio 2003 SP1 update on July 27, 2004. The full list of fixes and changes can be found here:
http://support.microsoft.com/?kbid=840663

According to Microsoft, Visio 2003 Service Pack 1 contains significant security enhancements, in addition to stability and performance improvements.

You can get the update through Office Updates (recommended) here:
Office Update Site

...or...

You can download it directly here:
Visio 2003 Update Direct Download