Yes u can set up a proxy with out intervention from the client side, it's called an inline proxy often transparent to the user. I would recommend SQUID it's pretty much the defacto open source proxy out there. There are other good ones, but this is one of the most widely used. Basically you will want to proxy all traffic from the machine. It's been a while since I worked with a PoPToP server but you would use IPtables to feed the traffic through the proxy server. This http://www.dataparadis.net/osp/gnu-l...-pptp-traffic/
will start to give you an idea of what has to be done with the traffic via IPTables. This won't be the easiest thing in the world, but it won't be the hardest either.