I'll tell you what the best solution to this is.....
Get UltraVNC and let me explain why.
With UltraVnc's "Single Click" add on you will be golden. The problem with most VNC's is that you need to have your customer open up ports on the router and firewalls. With Single Click , that does not have to be done on the client side. Secondly , you don't want to have to walk your customers through installing a program (like cross loop), its just a pain. With Single Click , you can make a CUSTOM .exe (Can fully customize the look and feel with your logo or w/e you want) that will AUTOMATICALLY install itself without the customer having to do anything , and remove itself from there PC when you disconnect.
All you would have to do is customize your SingleClick , and put it on your website. When you need to do a remote session , direct the user to your website to download it (usually a small file , the one I use with my company is 200kb). They just hit the connect button in the program , and your connected.
Best part , its Absolutely free, so go try it out!!!