Just a little list of what I'm using when I need remote access or remote control.
On the top of the list is Microsoft's Remote Deskop. Besides the accessibility (included in standard Windows XP installation), it is also blindingly fast, especially on DSL connections. Even more, it still works fine on dual ISDN connection (128 Kb/s).
What I hate about it is that it messes with my desktop. I'm running dual monitor configuration and everytime I connect remotely to my computer, RD moves all my desktop icons to the primary monitor. Yuck! (A small shell extension named Desktop Icons Save and Restore helps me restore them, but still - Yuck!)
That's why I sometimes use TightVNC for the same purpose. Not so fast, not so good (sometimes the screen is not redrawn when it should be), but it works quite fine on very fast networks; for example on the internal 100 Mb/s Ethernet.
When I'm using notebook and main computer at the same time, I usually use Synergy to control both machines with one keyboard and mouse. Read more about it in Steve Trefethen's blog.
Most rarely used of the great four (and the only one that is not free) is the MaxiVista. It allows you to turn additional computer (a notebook, for example) into an additional screen for your main computer. So when my two displays are not enough, I place the notebook on the table next to them, fire MaxiVista client on the notebook, MaxiVista server on the main computer and configure screen options to use the third monitor. MaxiVista handles image transfer over the network very well and most of the time I don't even notice that the third screen is not physically attached to my main computer.