suddenly no automatic connection to preferred network

I use my laptop at work and at home. The wireless networks at both places are
in my preferred networks list. So when I get to work from home and vice
versa, I have a smooth transition in connectivity. Since I have wireless zero
configuration enabled, my laptop (802.11G internal adapter) effortlessly
recognizes the WEP-enabled base stations at work (802.11G, belkin) and at
home (802.11B, MN-500), and connects appropriately without any further user
input (since the WEP keys are already entered).

Suddenly I have this weird problem. When I get home, I get this message on
startup: "unable to connect to preferred network". Upon looking in the
network connections, I see the SSID of my MN-500 and I click "connect" - and
I am asked for the WEP key. When I put it in, it connects without any further
problem. But it *does not* do this automatically as it used to; moreover, I
have to do this every time I reboot for whatever reason.

I cannot help thinking that this may reflect some larger, more serious issue
which I have been able identify yet. I would be obliged if someone could
comment on this. I am running Win XP SP2, with 1 gig of RAM and 100 gig HDD.

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