I need a tool I can support users remotely with

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I have an excellent computer consulting business that, for better or worse, takes me far and wide geographically. I often have to burn up extra time and gas on simple things that I could actually do remotely. So I need a system that will allow me to remotely manage my clients' PCs, with this important restriction: it can't require any previous installation of any sort of client software - I need to be able to "push" it onto their desktops. Any suggestions, preferably reasonably priced, are appreciated.