Wow, you are a Saint!!
And I am glad you are comfortable in doing all this research for them.
To be honest, I have not done this, but knowing that virtualbox is an application, and applications can be started when the whole system is booted (see Preferences > Sessions > Startup Programs tab), and a virtualized machine can be left in a "running" state, I can see where this is do-able.
You should also be able to do this with Mint, as Mint 6 only requires 256megs of ram to run (after it is installed) so you could assign more ram to XP to enable it to run with speed.
However, even after saying all this, I would hope you ask the people using this machine to give Linux 2 weeks to see if they can duplicate everything they need. It would be safer, faster, and be less flaky. Wine's improvements in the last few months has been marvelous, so even some Windows apps they may "need" may run. Maybe they will do some "work" so you won't have to!!!!
Malachi 3:10 NRSV
...says the LORD of hosts... I will not open...windows...for you...
Linux user since 2001 pda; 2006 lappy