I have 860 Japanes std rom, and running qtrdesktop to my XP pro wirelessly full screen 604x480 graphics are extemely good - very sharp!
Some questions you have already probably tried.,..
1.) Are you allowing remote desktop connections on your XP machine
2.) Are you connecting via LAN or WAN - if WAN, is your firewall allowing traffic over the 2 ports - 80 and one other I can't remember offhand.
Give us more info on your problem/symptoms, and might be able to help..
Is qtrdesktop installed on your 860 OK? You could try connecting to MY XP pro box over the Internet - private email me if you would like to try and I'll give you the IP address.. That might narrow down the problem to a handheld or PC config trouble.
Dave