I assume you are using your Raspberry Pi as a HTPC by running Raspbmc on it. (Here is how to do that.) I also assume the Raspberry Pi is connected to your home wireless router or home network. Raspbmc can easily be controlled from any other computer on this home network.
In your Raspbmc, navigate to System → System info → Summary. Take note of the IP address (for example: 192.168.0.12) that has been assigned to your Raspbmc.
Navigate to System → Settings → Services → Webserver.
Enable the option Allow control of XBMC via HTTP. Take note of the port (for example: 8080), username (for example: xbmc) and password (for example: no password).
On any computer connected to your home network check if you can ping the IP address of your Raspbmc. For example, ping 192.168.0.12. You should be able to ping it, else check the settings of your wireless router.
Open a browser on the computer and access http://192.168.0.12:8080 The browser will ask you for the username and password. You should be able to control XBMC from the menu and by clicking the buttons on the klunky old remote control shown in the browser.
Tried with: Raspberry Pi Model B and Raspbmc RC3