You need following steps to do.
1.) Check if your freind is having subscription of static ip from ISP. Call the ISP to ask this. If Yes, you are 50% over , if no you need to register the with DYNDNS.COM and further register a dns entry in that account to track your freinds machine with dynamic ip. Put that dyn name ( example friend.dyndns.org ) entry as registered in the ADSL router of your friend under dynamic DNS option..
2.) In friends ADSL router there will be PORT Forwarding option ( may be under NAT , you need to look for it and location of that option may vary with version of adsl ). Forward port 3389 to LAN ip address of your friends computer.
3.) Enable Remote access option in freinds computer from properties menu of computer. Creat a password from administartor account or creat a new account with remote desktop prevelidges.
4.) Now at your machine in remote desktop give friend.dyndns.org ( or the name you registered in dyndns.com ) . It will work.
The second option is simpler to just create account in logmein . Sit on friends computer and open access logmein account . you will see the option to add a computer . Just add this computer. And you are done.
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