You cannot.
This is part of the idle time optomization of XP.
Other than the noisy drive, it should not effect your ability to use your computer.
You can force the optimization process, as well as any other processes that run during idle time, to start. Click Start and select Run. In the Run dialog box, type rundll32.exe advapi32.dll,ProcessIdleTasks (note that this is case-sensitive) and press ENTER or click OK.
Essentially what XP is doing is moving applications closer to the (faster) tracks nearer the drive spindle to speed up boot and access times. It is not doing a full defragmentation.
____________________________
Users Helping Users