The Free CCleaner should get you started. Don't pay for so called PC cleaners or optimizers as these have not been shown to be any more effective than freely available ones. They tend to be more marketing than anything else.
Most overall slow ups in systems are caused by unnecessary files clogging and fragmenting the hard drive. Let CCleaner delete these unnecessary files and allow WIN 7/8 to defrag the HD.
Most slow ups at start up are caused by unnecessary processes being loaded. CCleaner will also allow you disable the ones that you don't need, which tends to be most of them listed. Don't disable AV or firewall.
Make sure to also do an occasional checkdisk with repair on you HD.