You can certainly boot from an Ubuntu (or any other distro) live CD and make changes to your Windows XP registry. Since you are working directly with the files on the hard drive, there will be no restrictions to what you can do; you are not booted into Windows, so it has no influence.
The only direct ramifications to editing the registry in this way would be that XP will act according to those changes when you boot back into that OS. If you make a mistake, the system may respond with errors or, worse, the dreaded BSOD. If you enter the correct information, you should be OK. Note that CHKDSK will likely run on boot because of the changes you made.
Before you do anything to the registry, you MUST back it up. If you don’t, and something goes wrong, you’ll have no way to reverse the changes and restore proper operation to your system. This list shows the registry hives and their corresponding file locations.
<li>HKEY_LOCAL_MACHINE\SYSTEM –> \%SYSTEMROOT%\System32\Config\System</li><li>HKEY_LOCAL_MACHINE\SAM –> \%SYSTEMROOT%\System32\Config\Sam</li><li>HKEY_LOCAL_MACHINE\SECURITY –> \%SYSTEMROOT%\System32\Config\Security</li><li>HKEY_LOCAL_MACHINE\SOFTWARE –> \%SYSTEMROOT%\System32\Config\Software</li><li>HKEY_USERS\<security ID of username> –> \Documents and Settings\<username>\Ntuser.dat</li><li>HKEY_USERS\<security ID of username>_Classes –> \Documents and Settings\<username>\Local Settings\Application Data\Microsoft\Windows\Usrclass.dat</li><li>HKEY_USERS\.DEFAULT –> \%SYSTEMROOT%\System32\Config\Default </li>
At minimum, you should back up the files that contain the key or keys you are changing; I recommend that you back up everything listed above. You can use the GUI file manager in Ubuntu just like you use Windows Explorer to make copies of the files.
Create a directory as follows: \%SYSTEMROOT%\System32\Config\backup.
Then copy the entire \%SYSTEMROOT%\System32\Config folder into your backup folder.