Hi,
I'm not an expert but I am also trying to do this and have
found out a little about it.
Firstly you need to get a static IP address for your
server/pc from your ISP. Without one the remote computer
will not be able to find your server over the internet.
For an additional £10 per month on top of the line rental
BT will give you 5 static IP's, a domain name, variety of
email addresses and 50mb of web space. Their number to set
this up is 0245 6007020.
You can then follow a network set up wizard (find a guide
to do this here
http://www.microsoft.com/windowsxp/pro/using/howto/gomobile
/vpns.asp )
Hope this helps a little.
Ben.