This simply means that your file is missing or is corrupt. You need to replace it from a good one, you can do that from windows installation CD.

On Microsoft website, following steps are given to solve your problem :

1.   Start the computer from the Windows XP CD-ROM.
2.   At the Welcome to Setup screen, press R to start Recovery Console.
3.   If you have a dual-boot or a multiple-boot computer, type the number that corresponds to your Windows XP installation when you are prompted to select the Windows installation to log on to, and then press ENTER.
4.   When you are prompted for the Administrator password, type the password, and then press ENTER.

Note If the administrator password is blank, just press ENTER.
5.   At the C:\Windows prompt, type the following command, and then press ENTER:
ren c:\windows\system32\drivers\isapnp.sys isapnp.old
Note The steps in this article assume that you installed Windows XP to the C: drive. The actual location of your Windows installation may vary.
6.   At the C:\Windows prompt, type the following command, and then press ENTER:
expand cd-romdrive:\i386\isapnp.sy_ c:\windows\system32\drivers\isapnp.sys
For example, type:
expand d:\i386\isapnp.sy_ c:\windows\system32\drivers
7.   After the file is successfully expanded, type exit, and then press ENTER to exit Recovery Console.
8.   Restart the computer.