Well, if you dont have to worry about antivirus, get a trojan like netdevil, or metaldeamon, they work nicely with windows xp, you can also get netcat, that is a very sweet and powerful tool to use on the target computer inconjuction with a batch file to open up a port on target computer for you to telnet to and have total access. The command you must get netcat to execute on the target computer will be: C:\>nc.exe -L -p [specify the port] -d -e cmd.exe
The moment you telnet to the specified port, cmd.exe will execute and you will have full access to that computer. That is one way... The other will be a trojan or a RAT (Remote access tool). Cracking the password will also work, get Cain & Abel to do that, You have Cain on your computer, make a SMB connection to the other computer, steal the password, and use cain to install abel on the target computer, once you have done that, you will have access to his computer in his dos console.

Astalavista!