Managing XP Desktops on my LAN

This seems like it should be relatively simple, but I just can’t find a good answer! My company has approximately 275 workstations on our LAN. Rather than getting up and physically going to the user’s desks when they have problems, I’d like to find a FREE and SECURE way to be able to sit at my desk and remotely connect to the user’s Desktop. I’ve looked at Ultra-VNC – it does what I want, but my peer and Manager have shown an uneasiness with the security of this software. Is there anything out there that anyone can recommend that will allay the security concerns and still be no-cost? Is security a legitimate issue with Ultra-VNC? Any help would certainly be appreciated. Thanks!