Home Network PC Asking for Password when it didn't before!

I have my XP Pro SP2 desktop networked to a XP Pro SP2 laptop using
wireless through a Linksys Wireless Router.

This has been working OK until today , when , on trying to access the
desktop from the laptop , I get a Password request page entitled
"Connect to dtop" and which has two lines entitled "Username" and
"Password"

The "Username" is greyed out with the words "DTOP\Guest" also greyed
out.

The "Password" line is blank.

As this is a home LAN and I am the sole user , I have not used any
passwords anywhere and this is the first time I have had such a
request.

There is no such request when I attempt to access the laptop from the
Desktop and all shared files on the laptop are visible on the Desktop.

I can ping both laptop and desktop from each other.

Why am I suddenly having this password request appear in this
otherwise perfectly working LAN and , since I am obviously the
Administrator here , how do I gain access to the Desktop from the
laptop ?
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