Start ur computer in safe mode with command prompt and type the followinf command to enable registry editor:-

reg delete HKCU\software\microsoft\windows\currentversion\policies\system /v "DisableRegistryTools"

If XP is not installing from the CD or if you have a new drive with no operating system on
it yet try these:

Install Windows XP from the hard drive with Windows 98 already installed:

Boot Windows 98
Insert the XP CD into your CD reader
Explore Windows XP through My Computer
Copy i386 folder to C:\
Go into C:\i386 folder and double click on winnt32.exe to launch the setup from the hard drive

Install Windows XP from DOS (ie. no OS on a new hard drive):

Boot with a Windows 98 Start Up disk
Insert the Windows 98 CD into the CD reader
Run smartdrv.exe from the Win98 directory on the windows 98 CD (file caching)
Type cd.. to back up to the root directory
Insert Windows XP CD into the CD reader
Copy the i386 folder to C:\
Go into C:\i386 folder on C: and type winnt.exe to launch the setup from the hard drive.