I honestly never looked at stripping IP from emails, but I would like to think that most email systems, strip this information out (I could be wrong, [and I'm sure I am wrong. Nothing would really suprise me]). If anything I would think that you would find his ISP's WAN side IP. Which you could get just as easily with a lan connection monitor, or even with a Netstat -a from a command prompt. I can't cover everything, some of my knowledge is old-hat by now anyway, but here's some of what it would take.
Fingerprinting & dreaded Firewalls:
Things get a lot more complex with the number of firewalls. I guess you could read up on enumerating IP's, Port Scanning, advanced tracerte's. But, honestly it would be a bit of work for you and a lot of tools (netcat tools, packet sniffers (eg. ethereal), fingerprinting/discovery/portscan/ tool like (GFI Languard), etc... You'd have an easier time aquiring these tools for linux rather than Winxp.
Quicker Workaround (Social Engineering)
I guess you could have him run a tool like
FindmyIP. By convincing him your trying to get an online game to work. In the process you'll be able to extract other info, like if he's using a router and the like. That will take some of the legwork out of the way for you. And it's the first route I would take. You'll still need to gain administrative ,access to his OS, and or router depending on how it's set up. And although you say it's a gag on your bro, it's still illegal. And depending on where he is, you could end up breaking a lot of laws in the process.
It's just not worth it for a gag like this. Unless of course it's as easy as your initial question suggests.
And it's really, really not worth it to mallicously attack someone else, if it's not your bro we are talking about.
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Easier Method:
Instead have him download one of your DB pro projects. Hide a program inside that program that get's extracted to his startup folder during the install, or get creative with a scheduled task. This way it won't happen right when he runs your Pong demo. I don't think I would bother trying to code a Terminal Stay Resident (TSR) / Spyware / Virus in DBpro. Not sure if it's even worth it. More than likely you would wind up coding a simple assembler that interprets byte codes, and at that point you'd be better off coding it in Assembly to begin with.
I would stick to coding fun games!!!