Looking for a remote access program

I am looking for a good program to allow me access my clients computers remotely. Something that I can install on their PC and have access to working on their files and bookkeeping programs. Any help would be greatly appreciated.

any of these should do - i like ultravnc myself, but any should be fine

Thanks for the help. I may just run it through XP and see how that works for me.

i used to use xp's native remote desktop to do this, worked great...years ago i used pcanywhere.