"Normally after a reinstall it recognises it immediately, but no luck this time."
Hi Roger, have you installed the XP service packs and updates and what version is the installed service pack if any? If it is SP2 there is a known issue and a fix for it, post back with the SP version you have.
Also check for an updated driver for the network card or on-board network device.
"as soon as I install my sound card, booting doesn't get any further than the screen inviting me to 'press del"
If you do press Del does the system enter the BIOS setup screen or does it remain frozen?
Does the motherboard have on-board sound? If it does is it disabled in the BIOS? Can you test the sound card in another PC to make sure it is working okay? Try plugging the sound card into another PCI slot and see if the system now POSTs properly and boots.