I've got lots of P3/PC100/i810 systems, lots of Cel2/PC100/i810 systems and one P4/RDRAM/i850 system at work. I ran exhaustive virus scans of the hard drive (and timed them, of course) before and after installing IAA, to see what effect it had on these, and it really made no appreciable difference one way or the other. One of the potential gains of using IAA is that it can switch your Auto-Acoustic Management-equipped hard drive into Performance mode for quicker seeks (this will reduce seek time on a Seagate Barracuda ATA IV by about 1ms, for instance).