i had the same issues, so i got rid of the guest account.. i think it has to do with the guest account not having the proper permissions. i suppose you could dig around and set all the permissions if you wanted to, but why go thru the hassle? easiest solution for me was to give the xbox it's own user account on my pc.
go into control panel and select user accounts. next click on create new account. when prompted, name the account xbox. the next screen should be where you assign either administrator or limited status, choose administrator. after you hit create account, it should take you back to the main account selection screen. from there, click on the xbox account. from there, click on create a password. for the password, use xbox. once that is done, go ahead and close out your control panel.
now you want to make sure the folders you want shared are set up to do so.. in my case, i just shared entire hard drives.. go to my computer, and right click on the drive/folder you want to share. under the sharing tag, make sure sharing is enabled. you might want to write down the share name given, with a note saying what's in the particular drive/folder. after doing this with every drive/folder you should have a list like so:
h$ ... movies
i$ .... music
g$ ... dvd-rom of pc
next you want to open up your xboxmediacenter.xml file and create your smb bookmarks. this is pretty straight forward..
smb://(username):(password)@(computer's ip)/(share name)/
here's an example of mine, under the video tag..
CODE
movies on the pc
smb://xbox:[email protected]/h$/
pc dvd-rom
smb://xbox:[email protected]/g$/
the dvd-rom thing isn't neccessary, but i actually like it a lot.. saves the wear and tear on the xbox dvd-rom, and we all know how fickle those stupid things can be. also removes any media issues your box might have. for example, my old phillips doesn't like cd-r's so i can't read any burned mp3 discs.. take the same disc and pop it the pc drive, and i can now listen to it on the xbox thru the smb share. kinda handy..
anyways, i hope that helps out some..