Any recommendations for Remote Assistance software that does not require firewall modification for clients?

To assist client with software problems and perform training, we currently use a tool called Remote Helpdesk to connect to their computers and guide them through the process. This tool was pretty cheap (~$400 onetime for 3 support staff), and worked great - the client's PC actually initiates the connection to us, so there's never any firewall issues (vs. Remote Desktop, VNC software, or many other similar tools).

Unfortunately, the product doesn't work well with 64-bit O/S's and Vista in general (slows down by a factor of 10 or so). I am looking for alternatives that provide the same reverse connection capabilities to avoid firewall issues. The only solution I've found is WebEx's Remote Support, which is WAY too expensive ($449/month for us).

Thanks for all the assistance!