Make sure that your dvd/cd drive is the boot device, go into the bios ("F2" or "Del" key, or which ever key is shown for the bios) when in go to "boot" section and place your dvd/cd drive as first in the list, then reboot with your Windows disk in the drive and follow the instructions very carefully below.
If you get the error:
Windows could not start because the following files is missing or corrupt
\WINDOWS\SYSTEM32\CONFIG\SYSTEM or \WINDOWS\SYSTEM32\CONFIG\SOFTWARE
1.Insert and boot from your WindowsXP CD.
2.At the first R=Repair option, press the R key
3.Press the number that corresponds to the correct location for the installation of Windows you want to repair.
Typically this will be #1
4.Enter in the administrator password when requested
5.cd \windows\system32\config
6.Depending on which section was corrupted:
ren software software.bad or ren system system.bad
7.Depending on which section was corrupted
copy \windows\repair\system
copy \windows\repair\software
8.Take out the CD ROM and type exit
Do this below first, if it works then Ok. If not do the top one.
Repair Install
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.
1.Insert and boot from your WindowsXP CD
2.At the second R=Repair option, press the R key
3.This will start the repair
4.Press F8 for I Agree at the Licensing Agreement
5.Press R when the directory where WindowsXP is installed is shown. Typically this is C:\WINDOWS
6.It will then check the C: drive and start copying files
7.It will automatically reboot when needed. Keep the CD in the drive.
8.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)
9.When prompted, click on the Next button
10.When prompted, enter your XP key
11.Normally you will want to keep the same Workgroup or Domain name
12.The computer will reboot
13.Then you will have the same screens as a normal XP Install
14.Activate if you want (usually a good idea)
15.Register if you want (but not necessary)
16.Finish
17.At this point you should be able to log in with any existing accounts.
Hope all works out for you.
Please post back if you have anymore problems or questions.
