Raphaël Rigo wrote:

 > Dec 7 18:27:45 pici kernel: Disabling IRQ #18
 >
 > I am using a P4 2.6 Ghz without HT activated
 > 512 MB DDR PC3200 on an ASUS P4P800 Deluxe MB.
 > HardDrive is on SATA (native mode) : Maxtor 6Y120MO.

Sounds like the known problem with an interrupt storm with the ICH5
See "Issue #2" at 
http://www.ussg.iu.edu/hypermail/linux/kernel/0312.0/0597.html

 > 2.4.23 doesn't have this problem but keeps using 30-50% CPU even if I 
do nothing.

Try vmstat, I expect you'll see ~100k IRQ/s

You could try using the ata_piix driver (located amongst the SCSI drivers).

If that still doesn't help, you could try adding the patch attached to 
drivers/scsi/libata-core.c
this should make the ata_piix driver catch the spurious interrupts

    Jon