Two things to try:
1. Change your command line in the scheduler to this:
"C:\Program Files\iSilo\iSiloX\iSiloX.exe" -x "C:\WINDOWS\Desktop\extractions_wcc.ixl" -u
(changed to run off iSiloX vs. iSiloXC...on my system, w98, either one works...although in the back of my mind there's a reason why you'd want to use XC...)
2. What happens if you copy the command line and paste into the "Run..." command on the start menu? (not familiar enough with XP, but assume they kept the "Run..." off Start).
Again, that works for me, and will tell us whether it's a command-line issue or a scheduler issue.
Not much, but grasping for straws at this point!
