I had the same problem and nearly gave up. Mine finally worked when I added an account on the Roku PB with the username\password for an account on my computer, and then manually added the share. I created a new limited account called "roku" on my XP computer and did not use my admin account to log in to the shares.
1) Set up new account in WinXP called "roku" with a password of choice.
2) Enter this new account in to the Roku PB under Network Shares with the username specified as <computer name>\<username>. If you create a new account called "roku" as well, then you'd add an account to the PB with the username "windows-8ix5q5j\roku" (without the quotes, of course).
3) Look for new shares in the Roku. If your machine appears in the list, select it.
4) If the computer does not appear, manually add it:
a) Enter "windows-8ix5q5j" for machine name
b) The IP address of the WinXP machine should automatically show up. If it does not, enter it.
c) Re-enter your user-name ("windows-8ix5q5j\roku") & password here for this share.
d) Enter your share name ("SharedDocs" in your case) under share name.
5) After entering the share, try to select it. If you are able to select the just entered share, then you're set.
If you continue to have problems, try to eliminate all potential sources. One thing I did was to temporarily turn off all firewalls and then reboot my XP machine, and reboot the Roku. I also made sure that both the Roku & the PB were connected to the same router.
After the share was found, and everything worked, I started ZoneAlarm and verified that the share still appeared on the Roku PB. Then I added the network switch that I was using with the XP machine and verified again, and so on.