HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\ Explorer\VolumeCaches\Temporary Files
Changing the value to 0 will force the Disk Cleanup utility to delete all the files in the Temp folder every time that you select the Temporary Files check box.
Notes: Since editing the registry is risky, be sure you have a verified backup before saving any changes. This tip applies to both Windows XP Home and Windows XP Professional.
Links:
[1] http://linux.philosweb.com/drupal5/files/rtelnk_2_3.gif
[2] http://articles.techrepublic.com.com/5100-10877_11-6168868.html?tag=nl.e064