Vista - break statement in vbscript wont launch debugger

I've got a script host (implementing IActiveScriptSite) which I'm
using to run VBScripts successfuly but I need to debug them. I know
you can put a 'break' statement in the script and that will launch a
debugger (like Visual Studio 2005) which seems to work on some systems
around here but not mine!

Running the command line "cscript //x scriptname.vbs" where the script
has a 'break' raises a dialog asking me to choose a debugger and I can
happily step through the script. However I get no such dialog when
running the same script from my script host instead I get a messagebox
displayed by the script host's implementation of
IActiveScriptSite::OnScriptError().

Has anyone else come across this problem or have any info about it?
Is there another way to debug VBScripts via Visual Studio 2005 or
2008?


BTW all our development systems are the same ie XP Pro SP3 with VS2005.