A Thomson is normally 192.168.1.1 - unless its one of the old Speedtouch models supplied by BT
To the problem
You're using WEP
WEP can be either "open" or "shared". Its an option under the advanced settings tab in the windows wireless network setup tool. However often with third party setup tools you don't get the option and it defaults to "open" and can't be shared
Set the PC up to use the Windows setup tool - not the 3Com one and try using the "shared" option
Also if you can get access to the router via a cable, try to change the channel its running on. Some network cards don't like running on channel 1 or 13 - even though these are the only two channels which don't overlap into another
If that still wont fix it then press the reset button (usually hidden in a pinhole), to set the defaults and then reprogram it. Make sure you have the ISP user name and password before you try this (who is the ISP??)