If XP is corrupted to the point where none of the previous solutions get it to boot,
you can do a Repair Install that might work as well as keep the current settings.
Make sure you have your valid WindowsXP key.
The whole process takes about half an hour depending on your computer
If you are being prompted for the administrator's password, you need to choose the 2nd repair option, not the first.
Insert and boot from your WindowsXP CD
At the second R=Repair option, press the R key
This will start the repair
Press F8 for I Agree at the Licensing Agreement
Press R when the directory where WindowsXP is installed is shown. Typically this is C:\WINDOWS
It will then check the C: drive and start copying files
It will automatically reboot when needed. Keep the CD in the drive.
You will then see the graphic part of the repair that is like during a normal install of XP (Collecting Information, Dynamic Update, Preparing Installation, Installing Windows, Finalizing Installation)
When prompted, click on the Next button
When prompted, enter your XP key
Normally you will want to keep the same Workgroup or Domain name
The computer will reboot
Then you will have the same screens as a normal XP Install
Activate if you want (usually a good idea)
Register if you want (but not necessary)
At this point you should be able to log in with any existing accounts.