Easy VPN setup for remote users

Hi,

we have some remote users that need help setting up various tools on
their computers, and we would like to be able to do this for them via
VPN, having them for example downloading an application and starting it
so we can access their computer.

It is important that the procedure is as easy as possible for the
customer, and that it should work well even if the customer is behind a
firewall.

Do you know of any configurations that could work for this?  We have
access to a public internet server that runs Linux, maybe the customer
end application could connect to that?

Thanks for any suggestions,

Morten