Remote Assistance Program?

Clarifications

Yes the customer will have to add on a small/light program but that is it, from there you can have all the access that you want.
TightVnc:
http://www.tightvnc.com/

UltraVNC has a 'Single Click' app that allows user access without installing anything, or the full-blown app which works very well. Personally I use uVNC on all my PCs at home as well as for my security DVR. Since there is an iPhone app for VNC, (Mocha VNC), I can do anything on my home network on my phone.
http://www.uvnc.com/home.html

Hi you can try logmein or even teamviewer - teamviewer has a standalone non install option as well.
no need to reconfig the firewall with these

The easiest that I've seen is Techinline
http://www.techinline.com/
Would definitely recommend this one.

Check out http://join.me - The end user has to go to the site and click "Share" and give you/email you the number which is a little inconvenient in a support role but it isn't too hard to walk them through it over the phone.

You can also use the Cisco WEBEx,This the paid one you can avail the 14 days free trial......

Check out ScreenConnect. They have 3 methods to connect by and allow branding of the software. It isn't free but its priced competitively. They do offer a free 30 day trial. http://www.screenconnect.com/

Although for Internal, we use RealVNC.