This forces mailto: links to open in Netscape 6, though varying the command line should allow you to use other clients.
1. Run regedit
2. Go to: HKEY_CLASSES_ROOT\mailto\shell\open\command
3. Change the default value to: C:\PROGRA~1\NETSCAPE\NETSCA~1\NETSCP6.EXE -mail "%1"
(for Netscape 6. Other clients will have their own command lines)
Now sit back and avoid the viruses!