Well, I've installed the Hyperion drivers, and that made no apparent difference.
The Control Panel doesn't offer the option of changing IRQs on any of those PCI devices - I haven't moved the sound card from slot 3 to slot 4 yet, but will try that this evening.
Meanwhile ... I've ordered a new copy of XP Pro which should arrive in the morning (I'm beginning to have doubts about the one that came with the machine), but I'm told it's a "new install" copy and won't work as upgrade or fix. I remember hearing that before (was it Win 98?) and having to rename win.com to something else to fox it.
Is there a similar trick with XP, or will a "new installation" copy fix a damaged one? I certainly don't want to wipe the machine and start again!