1. You need to remove the BootManager loader from the secondary partition, and then use the BootSect.exe utility which can be located on your Vista DVD.*
2. You then need to select option /nt52 to restore the ntldr loader.
3. You then need to select option nt60 to restore the BootManager loader.
4. You will now need to edit the boot options which were previously located in the boot.ini file but no longer available on Vista. You will need to use the BcdEdit.exe tool to edit this.
Although you could remove Vista and then just edit the BcdEdit.exe tool and remove the boot file for Vista which I think would actually work, I am not 100% sure.
* Windows XP will be your primary partition because Vista will not dual boot unless XP is installed at the primary partition. Therefore the secondary partition will be Vista.
IF YOU ARE NONE TECHNICAL AND DO NOT UNDERSTAND ANY OF THE ABOVE TASK, THEN I STRONLY SUGGEST YOU DO NOT UNDER ANY CIRCUMSTANCES PERFORM ANY OF THE ABOVE. YOU PERFORM THE ABOVE AT YOUR OWN RISK AND ANY LOSS OF DATA IS YOUR RESPONSIBILITY.