I do have the same configuration - an administrator and a simple user - and I will do some tests and post them. See you round. 
Everything works fine to me in Windows XP, logged as a common user and have a 'scheduled' task (configurated by the administrator into avast Enhanced Interface) running:
* avast! Report
* This file is generated automatically
* Task 'Scan: local disks' used
* Started on Sunday, November 09, 2003 4:20:01 PM
Infected files: 0
Total files: 125034
Total folders: 4220
Total size: 17.2 G
* Task stopped: Sunday, November 09, 2003 5:06:04 PM
* Run-time was 46 minute(s), 3 second(s)
Maybe the access trouble is on the system file. I'm using FAT32 (I get tired of NTFS...). Maybe my results differ a little from doggeral's post... because I'm not talking about Windows' Task Scheduler...