Are u sure PPTP is what you want to do?
It sounds like all devices are connected to the internet using a broadband, always-on connection. If so, then your task is made really easy with VNC. I have accomplished this for a number of clients,using the following scenario:
Home PC --> Router-->No-IP.com Free DNS-->Router-->OfficePC
Step 1: setup free account with no-ip.com, or dyndns, or similar. Install DUC Client onto Office PC, ensure it is able to update itself. (it works by reporting to No-IP the external IP address of the router, being the unique IP address on the internet, and no-ip translates this to a url, eg myofficepc.no-ip.com)
Step 2: configure the work router to enable Port Forwarding of TCP packets on port 5900 to the internal network IP address of the office PC.
Step 3: Load a VNC application onto the Office PC ( I use UltraVNC 1.01). Configure with a password , not with MS-Logon.
Step 4: On your home laptop use a vncviewer, eg KRDC in KDE, to access the officePC at port 5900,eg myofficepc.no-ip.com:5900
If you're good, you will be asked for the password. Just enter this and you will see the remote desktop.
Hope that helps.