I liked AVG PC Tune Up (formerly TuneUp utilities I think), and the product is still very good, however now AVG have taken it over I am put off by the fact it puts up browser adverts for AVG products when you aren't using you computer for a while, something I dont expect in a paid for product. That and their very poor, and downright unfriendly, support line and I am put off upgrading. Given that though the product itself is nice and allows you to do things like disable programs being in memory till you use them (putting programs to sleep as Rodney74 says), optimise startup, clean up your cache etc, and clean and defrag your registry (though a lot of people say this is not that important now). Ive never had any technical issues with TuneUp (apart from the browser adverts) and it runs quietly in the background, and my laptops been extremely stable and fast with it.
Winutilities I wasn't that impressed with. I'm trying Glary Utilities at the moment and a few of the giveaway sites are offering giveaway licenses for this. It seems to have a nice interface and is fairly comprehensive. Its Chinese though, and I'm afraid I'm always dubious of Chinese software, but it seems clean and no adverts or attempts to load unwanted stuff.
Kerish Doctor I'm not familiar with, but others seem to like it, so it might be the best.
Most of these products have a try before you buy option.