I'm having similar problems. The Roku worked fine for a week, then it could no longer access shares on the computer I keep music and photos on. The two other computers on the network (wired Linksys) could still see the shares. Additionally, the Roku could still see shares on these two computers. All of the computers are running XP Home SP2.
The Messenger service was turned off on the music computer, but turning it on made no difference, and it was also turned off on the other two, which the Roku can still share to!
The only major difference is that the music computer is running Zone Alarm (turning it off makes no difference), while the other two run PCillin and Norton respectively. All have the Windows firewall turned off, although I do occasionally get messages that programs are being blocked by the Windows firewall. I wonder if this is another MS solution that is just a disaster in disguise?
I've heard some comments regarding the incompatiblity of Roku with Linksys, but in this case surely I'd have problems with all of the computers?
I like the Roku for it's ability to stream photos to HDTV, but if I don't get a solution to this soon, I'm going back to my Hauppauge. I can at least listen to music with that.