If you're using Windows XP *professional*, "Remote Desktop" is built in. In "System Properties", click on the "Remote" tab. Under "Remote Desktop", check "Allow users to connect remotely to this computer". Now other users (or yourself) can connect to this system, via [different] computers on the network. There is also third party commercial software available, such as PC anyWhere 12.0 [Win, Mac, Linux] from Symantec, and even web based services such as GoToMyPC, which you should definitely take a look at....
citrix|online : GoToMyPC
http://www.gotomypc.com