You should be able to boot either into safe mode or using a recovery CD such as the Ultimate Boot CD for Windows click here.
Once booted double click 'My Computer' and select 'Tools/Folder Options' select 'View' and change the settings by clicking on 'Show hidden files and folders' and unchecking 'Hide protected operating system files' then click 'Apply' and 'OK'.
go into 'Windows' and find the file called 'pss' open this and you will find 'boot.ini.backup'. Right click and COPY this file. Go back to 'C:' (where the list showing 'Windows, Program files, etc. is) and right click on a clear area. Select 'Paste'. Find the file called 'boot.ini.backup' you have just pasted and rename it 'boot.ini'.
Go to 'Tools/Folder Options' click on 'Do not show hidden files and folders' and check the box beside 'Hide protected operating system files' then click 'Apply' and 'OK'.
Reboot your PC and it should start normally.
If you are unable do do the above look at this click here or click here.
You can also create a new boot.ini using a Windows CD as follows but it does take longer.
Resolution for Invalid boot.ini file
Resolution for the problem
Resolution for this problem is not difficult, but for those who face this problem for the first time, it can cause a headache.
To resolve this issue, first start the computer from the Windows XP CD. Then, start the Recovery Console, and then use the Bootcfg.exe tool to rebuild the Boot.ini file.
To perform recovery, follow these steps:
1. Configure the computer to start from the CD-ROM or DVD-ROM drive. For information about how to do this, see your computer documentation, or contact your computer manufacturer.
2. Insert the Windows XP CD-ROM into your CD-ROM or DVD-ROM drive, and then restart your computer.
3. When you receive the “Press any key to boot from CD” message, press a key to start your computer from the Windows XP CD-ROM.
4. When you receive the “Welcome to Setup” message, press R to start the Recovery Console.
5. If you have a dual-boot or multiple-boot computer, select the installation that you have to use from the Recovery Console. If only one OS type `1` and press ENTER.
6. When you are prompted, type the administrator password, and then press ENTER. If no password just press ENTER.
7. In the command prompt, type bootcfg /list (note the space before the `/`), and then press ENTER. The entries in your current Boot.ini file appear on the screen. If no Boot.ini file is available a message will advise you in which case go to `8` below.
8. In the command prompt, type bootcfg /rebuild (note the space before the `/`), and then press ENTER. This command scans the hard disks of the computer for Windows XP, Microsoft Windows 2000, or Microsoft Windows NT installations, and then displays the results.
Follow the instructions that appear on the screen to add the Windows installations to the Boot.ini file. For example, follow these steps to add a Windows XP installation to the Boot.ini file:
a) When you receive a message that is similar to the following message, press Y:
Total Identified Windows Installs: 1
Add installation to boot list? (Yes/No/All)
b) You receive a message that is similar to the following message:
Enter Load Identifier
This is the name of the operating system. When you receive this message, type the name of your operating system, and then press ENTER. This is either Microsoft Windows XP Professional or Microsoft Windows XP Home Edition.
c) You receive a message that is similar to the following:
Enter OS Load options
When you receive this message, type /fastdetect (note the space before the `/`), and then press ENTER.
* Note that the instructions that appear on your screen may be different, depending on the configuration of your computer.
9. Type exit, and then press ENTER to quit Recovery Console. Your computer restarts, and the updated boot list appears when you receive the “Please select the operating system to start” message.
After these 9 steps have been performed, the problem with invalid boot.ini file should be solved.