The Op said that the system was tired, dirty fans etc..best to clean it first(!), but look at ATM's, supermarket tills, servers, all just PC's of one sort or another, always left switched on. Worst thing ever turning off an old hdisk or fan, and it never returning to life. Obviously if power savings are worse than component replacement, then don't do it.
Although saying that, I always turn mine off, it has a horrid high pitched whine, and is located in the bedroom. (computer AND wife!)