Cleaning up an unwanted dual boot is a lot easier in Windows XP than in Vista. XP's System Configuration Utility has the ability to delete dual-boot information for any operating systems that are no longer present automatically. In Vista, it's rather more of an effort, but you can still do it.
Click on the Start button and type in 'cmd' then right-click on it and select 'Run as Administrator'. Now, type 'BCDEDIT /enum' into the command prompt. You should see several entries, including one labeled Windows Boot Manager and two labeled Windows Boot Loader. The latter two represent the two operating systems. There may be other items listed as well.
Each item will have a number of lines under it including an identifier line, like identifier {current}. The identifier for the old OS will take the form of a GUID (globally unique identifier) like cbd971bf-b7b8-4885-951a-fa03044f5d71. Type 'BCDEDIT /delete {identifier}' into the command prompt, replacing 'identifier' with the GUID number.
