how to change %systemroot% from G: to C: - drive letters correct

I have a client's computer which is booting from his USB backup drive, which has somehow been marked as active and has Windows etc on it. I did not set up the drive, and as far as I know, he thought he was just backing up his computer. The drive has been in use for 11 or 12 months. Drive C: is the drive it should boot from, and drive G: is what it is booting from. Opening the Run command and typing C: opens the correct drive. Typing %systemroot% opens G:
He reports that this just happened two days ago, and if the USB drive is not connected, it won't boot. Is there somewhere in the Registry I can change the drive letter for the %systemroot% ? I don't think MS KB 223188 applies, because the drive letters are correct.
Ron-computertutorAsked:
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Dhiraj MuthaLevel DCommented:
As per my knowledge, when you trype %systemroot% by default it opens the drive where windows is installed. Its not Drive Letter specified.
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Krzysztof PytkoSenior Active Directory EngineerCommented:
Yup, %SYSTEMROOT% is a variable which point to drive where OS is installed. In your case, someone installed XP to G-drive.
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Ron-computertutorAuthor Commented:
Actually, XP was installed on the C: drive, but somehow system files have become replicated on G:, and the computer is now booting from G:. I know the C: drive is fact because I have had the computer in my shop without the USB drive G: in the past, and it booted correclty off the internal C: drive.
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Vikas ShahCurrently Seeking OpportunitiesCommented:
what happens if you remove the USB from the machine.
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Ron-computertutorAuthor Commented:
I just tried that, and the computer boots to the point where the mouse pointer shows on the screen (and will move), but no login. Just a small Windows XP logo in middle of screen (about 2" square). I have UBCD4win, and was just about to boot with that and see if the userinit file is where it shoudl be. I tried F8 and 'boot last known good configuration' and got same result as above.
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Dave BaldwinFixer of ProblemsCommented:
Look at the 'boot.ini' file in d0:p0, drive 0 of controller 0, and see what it says.  You may be able to change it there.  Also check the BIOS settings to see if they have changed the boot drive.
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Jerry MillerCommented:
The registry key
HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\SystemRoot

Holds the file path to the Windows install. Please be careful in the registry, it could render the system completely unusable. Make a bakc up of it before making changes.
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Jerry MillerCommented:
Yeah, Last Known Good is only a good option if it hadn't booted properly from the USB drive. Because now that is the 'good' one according to XP.
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Ron-computertutorAuthor Commented:
I checked, and the userinit.exe file was where it belonged. I used the Registry repair function in UBCD4win to restore to registry from Aug 2nd, when I believed it was working ok. Rebooted, and have control again!
I checked the boot.ini file, and it appears to be reading correctly now, although I don't know what it was befoer the change I just made. I checked the Registry currently, and it does show systemroot on c: as jmiller1979 suggested. I really appreciate the information offered, ane even though I solved it myself, I am going to award points to jmiller1979 and DaveBaldwin. I intend to go to the portable drive and open the Registry to see if it did say drive g: for systemroot, but everything seems to point that way.
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