It's quite easy, just send suspend and resume signals to the process:

sh$ kill -STOP $pid            # this suspends it
sh$ kill -CONT $pid            # this resumes it
[download]

You can stick that in your crontab. If there is a process group to suspend and resume, use -$pid. Pretty much, read kill(2) man page.

Update: I see this answer was given already, I did not read to the end of BrowserUk's post first time around (I gave up around "Windows" :))