The "Optimize Hard Disk when Idle" function is a partial defragmentation run. It reorganizes all startup and executable files according to the LAYOUT.INI file which is build up by PREFETCH. As far as I remember it runs every three days after 15 minutes of Idle time. There is no Task Scheduler entry for that, but the mentioned Registry entry should handle that (my favorite defragger UltimateDefrag offers to deactivate it for me and handles LAYOUT.INI himself, so I didn't check).
On Vista there is a default Task Scheduler entry running DEFRAG.EXE -C -I every wednesday at 1:00 am. It is set to run only during Idle (after 3 minutes) and will restart if it's been interrupted. It will be terminated after running for three days. Additionally Windows Defender is set to do a quick check every day at 2:00 am. Search Index does one big indexing run and from there on will only kick in whenever a file in one of the indexed directories is written to.
All background disc activity in Vista (Defrag, Superfetch, Search Index, Defender) runs at "very low" disc I/O priority and thus does not interfere with normal disc activity. That means that once your DAW needs to read or write data to/from disc the background disc activity is interrupted by Windows until the disc free to use again, just like with low priority CPU processes. The activity may lead to more R/W-head movement in your HD though and thus affect access times slightly. All DAWs and sample-based plugins use their own file-caching algorithms though and should not be very much affected at all.
Once that weekly defrag has finished (or is turned off) and Superfetch (which reads ahead from disc whenever there is free RAM available) is finished fetching files or turned off there is no considerable disc-activity happening on my system anymore (Vista 64-bit atm) apart from a few sporadic I/Os when processes like Explorer access the Registry and Search Indexer checking for updated files (can be turned off) and system log-files being written to.
One other source for disc activity can be automated Windows Update and any software/drivers that may automatically search the internet for updates. Antivirus applications like Norton do a "System Scan" by default after each update! So if you suffer from constant activity you may want to check those first.
You can use Vista's "Performance Monitor" to watch which processes produce CPU, Disc, Network and RAM activity including the priorities and amount of activity.
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