Don't know about XP, but both Windows 7 and Vista start doing some maintenance/tweak stuff like defrag/rearrange after a while the comp has been idle.
For example, this is just a part of the things that start in Windows 7 (no idea which use HDs a lot):
AnalyzeSystem, This job analyzes the system looking for conditions that may cause high energy use, Idle for 5mins
AitAgent,, Idle for 3mins
Autochk, This task collects and uploads autochk SQM data if opted-in to the Microsoft Customer Experience Improvement Program, Idle for 10mins
CrawlStartPages, Index all crawl type start addresses, Idle
Defrag, This task defragments the computers hard disk drives, Idle for 3mins
ProgramDataUpdater,, Idle for 3mins
SR, This task creates regular system protection points, Idle for 10mins
VerifiedPublisherCertStoreCheck, Inspects the AppID certificate cache for invalid or revoked certificates, Idle for 3mins
WinSAT, Measures a system's performance and capabilities, Idle for 10mins