on demand remote access

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Hello
I am looking for an on demand remote software like NetOp On Demand Remote Control. I run a consulting company and need be able to access the clients machine for tech support. I have been using vnc or rdp. But firewall setup and software install issues make this less than desirable.
As my client numbers grow I would like an on demand solution but I don't want to invest a ton of money right now.
Hoping that I could get some ideas.
Thanks in advance.
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