just install the patch for Exchange 2003 SP2 and reload the updated S/MIME control. While it doesn't require a reboot the store is stopped so your users will experience an outage.

http://msexchangeteam.com/archive/2006/10/23/429281.aspx

It's important to note that instaling the patch on the server alone does not solve this problem, the client must also be updated.. If you use SMS or some other software push mechanism you can push and execute setupmcl.exe from the update and clients will be fixed. The new control also works with IE6 so no need to detect which browser is in use.