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.

Well... I have had a similar problem. I was using blind (open) shares on my XP Pro box and that work ok after I got the kinks work out, but I moved all my music to a real fileserver running a server OS (windows 2000 server) and that is where the fun begins.

I can add an account for my server, and the roku sees the shares but does not let me get into them to play any music. So here is a hack and a possible fix for everybody to enjoy...

First the hack - at the command line I was able to type:

smbmount //server/musicfiles /mnt/smb/server/musicfiles -o username=account,password=actpassword

...and this of course mounted the files. I verified by going to the path /mnt/smb/server/musicfiles and doing an ls to see all my many music files and directories. Keep in mind that the directories server and musicfiles must exist in the path /mnt/smb/server/musicfiles for this to work. If they do not exist you must create them. They were already created for me by the system - probably because of my account creation step above.

Now the fix - it has been said that the above hack is not user friendly because it may not remount on reboot. That is easily fixed with a quick edit of the rc.local file. For those who don't know, this is an easy way to add stuff for the system to do AFTER everything else is started and running. That is exaclty what we will do. You will have to do the following:

Log in to your system
cd /etc
vi rc.local
use the arrow keys if working or the old fashioned ctrl-m to move down or even use :999 to get to the last line.
Then open a new line with 'o' and this will put you into insert mode.
Now paste or type the same smbmount command that we used before
Do a quick :wq to write and quit and you are all set.

Now with any luck you will get to your share all the time even after a hard reboot...

I don't guarantee this to work for everybody but it should help you guys out a bit. Command line tactics are very helpfull in the war against GUI's that don't always behave the way you want.