Some options for you to consider:
1. If your clients are Windows XP based, you could use the built-in Remote Assistance feature. This is fairly easy to configure, but so far as I know, lacks the facility to transfer files.
2. Ultra VNC is free of charge. It would require a once-off installation at the client, but it also supports file-transfer and disabling the remote keyboard and mouse. Find it at http://ultravnc.sourceforge.net
3. FogCreek Co-Pilot. These guys have taken the VNC engine, and written a service around it, which makes the installation and use much simpler for client and technician. There are many flexible payment options based on how you use it. Find it at http://www.fogcreek.com
4. Famatech Remote Administrator. This requires a license for each client machine but has to be one of the best, full-featured, easy to use, and fast systems out there. It can do file transfers, remote telnet session (without the Windows telnet service running), remote reboot, text (voice?) chat, and best of all, can proxy through other RAdmin servers, which is great for NAT, or other complicated firewall setups. Find it at http://www.famatech.com
VNC is the most flexible in terms of the number of OS's it runs on, but it suffers slightly in terms of performance and features.
Hope the info is of some use.