Enabling encryption preventing laptop wireless internet access - help please!

I have had a wireless network set up between a laptop and my PC which has
worked smoothly for a couple of weeks. Today when I booted up the laptop I
got the message "Windows was unable to find a certificate to log you into
the network " on top of my sys tray icon which now has a yellow triangle and
exclamation mark on it!

I browsed about and discovered this was something to do with encryption. Up
until this problem occuring I hadn't changed any of my router's (SpeedTouch
585) settings whatsoever or on the laptop. I tried putting in a brand new
WPA-PSK key into the router then put it into the laptop, but still the same
problem happens. I don't know is this is related but when I input the new
key into the laptop in Wireless Network Connection Properties>wireless
networks>properties>association hit ok then return to that window it has
gone back to the router's default key, a 10 digit key which doesn't work. I
also tried to repair the connection but that hasn't worked either.

I also read that if, in the network properties>authentication, the "enable
IEEE 802.1x authentication for this network" box is enabled it can lead to
connection problems. But it is unchecked. In fact, even if I wanted to
enable it I can't because the entire box is greyed out. Can someone verify
if this is where I can fix the problem, how this window came to be greyed
out and how to fix it.

If I disable encrytpion on the router the laptop can connect to the internet
no problem. I've tried a few keys, all generated by the online generator at
www.grc.com, and it still won't connect with encryption enabled. The
"certificate" notice just comes up again.

There are no viruses or malware on either machine, no new programs have been
installed just prior to the problem occuring

So how has this problem occurred and how do I fix it?

Help gratefully received.

Harry

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MSI KT4 Ultra
AMD XP 3000+
Thermaltake Venus Silentboost
Crucial 2x512mb PC2700 CL2.5 DDR RAM
Gainward Geforce 6800 GT (2400 TV-DVI Golden Sample)
Pioneer DVD-106
Plextor PX-716A
Western Digital Caviar 7200 WD3200JB ATA 100 320Gb (Master)
Western Digital Caviar 7200 WD2500JB ATA 100 250Gb (Slave)
Creative SB Live! 5.1
Alcatel SpeedTouch 585(i) Wireless ADSL Gateway
Firewire Card (generic)
Realtek RTL8139/810x Family Fast Ethernet NIC
Viewsonic VX912
Thermaltake Xaser III Super Tower
Thermaltake 420W Silent PurePower Dual Fan PSU
Win XP Pro SP2