|Do you have an domain administrator user that psexec can use on all the machines?|
Ok, so what we are going to do is use a for loop to loop through an input file, and execute your psexec command using information from each line. Since we have two variables that are changing for each iteration, we will have to create the input file in a special way. Will it be a problem for you to generate an input file that has this information on each line?
If not, then we can tell the for loop to look at the comma (,) as a delimiter, and it will therefore see machinename### and domainuser### as two separate tokens, which will allow us to create your command in the for loop.
If creating a single input file with the information as formatted above is a problem, we'll have to go a different (more difficult) route.