On the machine where it says s := Sys.Process('EXCEL',2), it looks like there is already an Excel app running. Or check Windows Task Manager and look for an Excel process that might not have terminated properly. Or just restart that Windows XP machine where it fails, and make sure that no Excel app is launched before you run your test.
An additional note: TestComplete uses the Index parameter to differentiate among processes when more than one process with the same name is launched in your system. Please see the "Sys.Process" help topic for more information.