Depends on what you are doing, also some of the free remote desktop apps do not support encryption. Keep that in mind.
I don't know if remote desktop from windows supports encryption either but I messed around with open vpn to try to get one to work.
I found VNC viewer to be good, www.realvnc.com
real vnc is good. i personally like tightvnc viewer just because it is compatible with my ipod touch! but either of them are good