If you want to do this the Microsoft way, only using software that comes with Windows XP, you can enable Remote Desktop on your PC and set up port forwarding on your ADSL router to forward incoming connections on TCP port 3389 (RDP) to your PC's internal IP address.
For an additional layer of security and encryption, you can set up your PC to allow incoming PPTP connections and instead of forwarding port 3389, you set your router to forward TCP port 1723 (PPTP).
I use the PPTP method with MTN and it works with the standard APN. With Vodacom you need to call them up and ask them to provision access to the 'internetvpn' APN and change your configuration accordingly. I haven't been able to get this working with Virgin at all.
Be aware that you will be exposing your computer to the internet, be sure to use a good password.