vnc is good - but over the internet it is pretty poor.
remote desktop, as previously mentioned, is much better.
The main reason for this is that it compresses the signal between the two points. VNC doesnt do this, which is why over the internet and slow networks it is laggy.
If you run any version of win xp above pro, then you will have this option available on your pc. To set it up, go to control panel - system and click on the remote tab. In here click in the 'allow users to connect remotely' check box and your away. Now anyone in your network can connect to that pc.
You will need to open the rdp port in any firewall you have installed - by default it is 3389. If you want to connect from across the internet, then you will need to open this port up on your router and forward it to the ip address of the pc you want to control (or just open the port if there are many pc's you want to do this with). And hey presto, you will have a very usable remote desktop that you can use anywhere. Btw the client for remote desktop is in start - all programs - accessories - communications - remote desktop connection. Alternatively, you can go to start - run and type in mstsc to start the connection (or type it any where on the command line). using mstsc /console will actually give you admin access to that machine.
If you really have to use vnc (it isnt as good as remote desktop by far), use ultravnc. This version is also free, but actually installs a set of video drivers for you to compress the connection and make it at least a little more usable. Tbh, i dont think anyone uses vnc when remote desktop is available anymore - it even comes packaged in most linux distros now!