The Remote Desktop Connection feature in Windows Vista Business, Ultimate, and Enterprise editions enables easier remote access to any resource or application that your organization has made available to you.Vista Home editions doesn’t have remote desktop support.
First you need to right-click the Computer icon and choose properties.Now you’ll want to click the Remote Settings on the left hand side
Now you have to choose “Allow connections from computers running any version of Remote desktop” option and click on ok this option for for connecting windows XP/2k machines.
If you want to connect another vista PC you need to select “Allow connections only from computers running Remote Desktop with Network Level Authentication” and click on ok
For those who are experiencing problems with access Vista from XP computer, try to update the client software (In XP the version is 5.1 and current available for download is 6.0).Download From here