accessing a computer knowing IP addres

Just by knowing a computer's ip number does not permit any form of information sharing... It is nothing to find the ip number of a computer. The computer in question needs to have sharing enabled, and you need to have authentication.
You didn't mention if this computer is on a lan or on the internet. On a lan will be no problem if you have sharing and user rights, accross the internet is another story. The other computer will have to be using an FTP server to transfer via ftp, or host a dial in connection for you to connect to, or run host version of pcanywhere. in breif, just knowing an ip, doesn't provide anything.
Don't know if this answered you, but your question was breif! Lots more to consider than what was asked...

If the computer is MS OS, "net view \\IPADDRESS" will show you if any resources are open.
net use * \\IPADDRESS\resource_name will map your next avail driver to the victim computer. If it has authentication enabled, it becomes complicated. If not, you are free to add, delete and modify content (I'd suggest not to do that).
To see if the machine is running an MS system, scan for TCP/UDP ports 135-137-139.
Most older win95 systems are open to this exploit as the Microsoft NEtwork CLient and MIscrosoft Sharing services are bound to the DIAL-UP modem or the Internet access device.
DO NOT DO damage...
Regards,
Radu