Depending on the remote software you are using, you can practically do anything the user could do while sitting at thier desktop. They need to present to do things like push buttons, login to the desktop, put cd's in maybe...the obvious things you can't do remotely.
Vista poses a problem though because of the UAC "User Access Control". It must be turned off because for one you can't see it. this problem can be dealt with a couple of ways.
One is walking a client through how to turn it off or transferring a simple reg file to the remote system and merging it to the registry. Either way you will likely have re-stablish a remote session if it locks up or something.
Before you ask, I am using Crossloop, which is currently free. It uses VNC as the backbone and can transfer files, change direction of the share so a user can see your desktop also if need be. The only thing it doesn't do is chat. but I prefer the speakephone anyway since it also requires a broadband connection to get performance.
Radmin works good if you are on broadband and the client is on dialup but is $35.00/seat unless you buy multiple licences. Also is much harder to initiate a session because the user need to possibly configure the router to forward ports and know what thier IP address or router ip address is at the time of service.
Crossloop provides the easiest and cheapest solution I've personally found.
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