I don't know about XP, not used that for donkeys but since Vista is pretty much like windows 7 (which I run) maybe you can try a couple of other things.
1) Most common problem I see with people running Vista/7 and can't connect is they have not turned off password protected sharing, it is an absolute must.
2) This is not something i'm sure will apply to everyone but I do it, I have created a 2nd user account on my PC, I also gave that account a password. I took complete ownership of my PS2SMB folder for the PS2SMB user. In OPL, for the username & password I use the name 'PS2SMB' and the password I set for the account. Works all the time, I have never used the 'GUEST' acount so I dunno about that.
I'm pretty sure I've had password protection off on my Vista. But I'll try creating that specific account. Thanks for the tip. :)
Creating an user-account for OPL only works for revisions, where you can use other usernames, then the GUEST-Account.
This only applies to rev's (non-released beta-versions as of yet), which support 'protected sharing'.