An external travel HD is a good idea. DVD's play without adding files. I like the idea of booting to another OS. You can even find - by searching around - an XP os that can be installed on a flash drive and booted. There are many applications that are portable, that don't add files or any entries to the registry.
About the only case that would be impossible is if the laptop was loaded with something like Pointsec. In which, you're extremely limited in your options to use the laptop. Also, some laptops installed with certain VPN software, might require that all Internet access be directed through the corporate gateway and only specific software could be run from the laptop.
Best to check with your IT people.