There are two methods to use schtasks. One is by its API and the other is by the schtasks.exe. For the latter, put it and the command line parameters string in a Shell() command. For the former, that can be rather involved.

If you use the latter, try putting the string for the Shell() command into Start > Run, to make sure that the string is correct. The usual problem is that a string with space characters was not enclosed with quotes. You can also test using that string in a BAT file. Once you have the string built properly, you can build the string in VBA for Shell().

The best source for both methods is: http://msdn.microsoft.com/en-us/library/aa383608

There are various other API examples.
1. http://www.tek-tips.com/viewthread.cfm?qid=356060
2. http://www.andreavb.com/forum/viewtopic_1507.html