Ok, for Windows XP, make sure your 'Plug and Play' service is set to Automatic and is started. If that thing isn't running properly, it'll cause all sorts of havoc (usually more with sound cards, but also with network cards). Also check that the 'Network Connections' service is started and running.
And, just to confirm, you have no yellow (!) or (?) and no disabled devices (red X) anywhere in device manager. (Not just not under the "Network Adaptors" branch, but none ANYWHERE, right?) Is a 'Network Adaptors' branch even being displayed? If so, what's under it?
And, a next debugging step... If you still suspect software, as Zac67 said, try a BartPE or Linux Live CD. If that detects a network card properly, you know it's software. Might be time to reinstall Windows.
If the live CD does NOT detect the network card, though... It might be time to start examining why the hardware isn't being presented to the OS. (Probably a hardware fault or misconfiguration.)