How to Fix the Problem of XP Home Rebooting at Login Screen?
When you start your computer or restart it after a long session, you find that the XP Home reboots in a cycle denying you access to the desktop. The computer is able to get to the Windows startup screen but before it displays the login screen, it gives a light beep and reboots. The process keeps on repeating until you use a bootable CD.
The very fact that you need a bootable CD means that your computer is missing some file(s) or some system file(s) are corrupt. In case of Windows XP Home Edition rebooting, it is most likely the kernel32.dll file. The file is required to start the Windows Explorer process. As it is missing or corrupt, your computer is unable to start and keeps on rebooting. The following paragraphs explain how to get the computer back to a stable state.
If the problem started recently - after a long session where you removed a program - you can go for a rollback. Start System Restore and try to restore your computer to some most recent restore point. In most cases, the problem is fixed using system restore. If the rollback does not stop XP Home from rebooting at login screen, you will need to go deeper into troubleshooting by using the recovery console to replace the corrupt kernel32.dll file.
Start your computer using the Windows XP installation CD. Once you are at the screen where it asks if you wish to use Recovery Console, press R. You will get DOS like desktop with command prompt shown as "\>" without quotes.
NOTE that you need administrator privileges to run the recovery console so keep your admin password ready. Also, if the computer had multi-boot, you may be prompted to select the partition where Windows Home is installed. Make sure you select the correct partition. The prompt is self explanatory so you will not have difficulty in selecting the correct partition. The follows these steps:
 Once at \>, type CD system32 and hit Enter.
The command CD helps you reach the system32 folder. You need to move to the System32 folder as it contains all DLL files, including kernel32.dll
 To see if the file exists, type DIR KERNEL32.DLL
If the file is corrupt and exists, you will get the some directory information followed by the filename else you'll get a message that says something like "File does not exist"
 Type MAP and press Enter.
The MAP command gives you the drive letter of the CD-ROM, which is not the same when you boot with the CD. In this case, we assume you got an answer something like: E:\device\CDROM0. Make a note of the drive letter, which in this case, is E.
 Type EXPAND E:\i386\KERNEL32.DL_
Use the drive letter you got with MAP command (Ref [D]) in place of E. Also make sure that you include the underscore character (SHIFT+MINUS) after KERNEL32.DL else the command will fail.
Note that upon successful copying of the Kernel32.dll file, you'll get a message saying "One file(s) expanded".
 Type EXIT to restart the computer.
This fixes the problem when XP Home Reboots at Login Screen repeatedly. For more information on XP startup and shutdown problems, read Troubleshooting Windows XP - How to Fix Windows XP Problems.