FileHippo Update Checker is good software.
You can have to run on start-up and if no updates are found, the program will close. Since it installs no Services (ie. background processes) it is light on resources. If updates are found it will open your default (or chosen web browser from the settings) to display the results, so you can download and install them from FileHippo.com.
You can add other areas of your hard drive to be scanned if they are not located in the Program Files (ie. Portable Programs). It also can notify you of available Beta software if you want.
Since it does not run in the background like Secunia PSI, it won't auto-update software such as Flash Player.
There are a few settings in Secunia PSI for monitoring programs and whether you want it to run all the time.
I use FileHippo Update Checker over Secunia PSI, since that's my personal preference.