I'd recommend going ahead and installing Windows 7 64-bit, and running your Cisco software in a VM.
Edit: I just noticed joeqwerty pointed out that Windows XP Mode might not work very well for VPN software. I'm not familiar enough with Windows XP Mode to know why this would be a problem, but you could still use a different virtualization platform, such as VMware or VirtualBox.
For example, you could use VMware Server to install a 32-bit Windows XP virtual machine, and configure that VM to start up every time your computer boots up. As long as you configure the network adapter as a bridged network adapter, the VPN software should be able to route traffic between the local network and remote hosts, just as it would if it was installed on a physical machine.