How would I go about reinstalling a missing dll?

You can do it this way, but the original XP disk may not have the correct version of the dll required by whatever software on your system needs it. dlls are constantly updated with new versions but the same filenames.

You don't give enough enough info on what is alerting you to the fact that you have a missing dll.

if you get this message only when you launch a specific piece of software then uninstalling and reinstalling the software may solve the issue.

If you are getting the missing dll message from Windows system itself then if you are running XP SP3 [as you should be] but your XP disk is not SP3 then the dll on the disk may not be the correct version, but you can try and replace it.

Windows system dll's are stored in the System32 folder [in C:\Windows directory] so thats where you need to copy it too.

This will assist you in all dll related problems..visit the link.

Will help you in registering dll files.

http://fix-all-dll-errors.com/contactus.asp

you EXTRACT it from its compressed version

you then COPY it to the correct folder, using

SAFE MODE command prompt.

but it may depend on exactly which one it is.

SFC/SCANNOW

can also be used from the comand prompt to refresh System Files from the cache.