Zoe, before you run out and buy a new HDD ;) A HDD error does not tell you that your PCI,sys file is missing or corrupted. Check things first, it usually means that the actual file system System33\DRIVERS\pci.sys is really corrupt or missing. It is possible that there is a problem with a piece of hardware that is connected to your computer, this is common with a new hardware install, or a new driver. Check on here to see what Microsoft has to say.
You can attempt to repair this file by starting windows setup using the original setup CD Rom.
Select “R” at the first screen to start repair.
Make sure that you do not have any new hardware plugged into your computer. If the file is missing or corrupt you can replace System33\DRIVERS\pci.sys by restarting your computer with your Windows XP disk in it and choosing to repair.
If you can start Windows, try safe mode, simply paste the file back into the Windows/system32 directory. Even if it is still there, it may be corrupt. Of course, you can also choose to install a new copy of Windows which will cause you to loose all your previous data, but it will fix the error for sure. Hope this helps, good luck.