There are several ways to control Media Center without having to remote control the whole machine.
With the WebRemote plugin in the DevZone, you can control MC through a web browswer from anywhere. The control is somewhat limited, but it may work for you.
Alternatively, you can use the freeware Girder to send MC_COMMAND messages to MC9 or to access its Automation SDK. Then, you can trigger Girder events remotely from your laptop. This requires much more setup, but it may give you control of some things that WebRemote doesn't.
A third party tool called Glissando also can control MC9 from a remote machine, with a flash-based UI, I believe. I don't know much more about it, but it seem like it is or was hosted at AVSForum.com.
Finaly, the NetRemote project allows you to control MC9 from a network-attached Pocket PC. I don't think they have a desktop Windows client, but you could experiment with running it in a Pocket PC simulator if you are very motivated.
For remote control of the whole PC, nothing beats Remote Desktop Connection, built into Windows XP Professional. Otherwise, WinVNC works perfectly, but performance is lousy unless the machine being controlled is running at a low resolution and at a video bit depth of no more than 16.