Sorry, my post was a bit confusing. I didn't want you to use the sleep-plugin. I don't have neither a solution for you problem nor a suggestion. I just pastet MCs system info so you can see how my system is configured, because it goes into sleep mode with MC installed and also with MC open.
I just could give you some off my thoughts about this:
Vista will not go in sleep mode, if MC is playing.
Perhaps it also maybe a problem if MC builds thumbnails or is doing some other jobs, but I'm not sure about that. Some time ago I read about some problems with the detailed energy-saving settings concerning the setting "When sharing media".
(http://www.ocmodshop.com/images/guides/vista_tips/restore_sleep_mode/thumb_3.jpg)
This shouldn't effect MC, but perhaps it helps to set it to "Allow the computer to sleep"... You can just give it a try!