I used to transfer files quite easily between my desktop (Vista) and wife's laptop (XP) over our home wireless network.
From about 3 months ago whenever I try to access a folder or transfer a small window pops up asking for a password.
Because I never set it up for a password in the first place I don't have a clue what it should be. Pressing enter didn't work, maybe next time I'll try password- can't remember if I did already

John