LAN connection not being made on boot

Running XP Pro SP1 on my desktop PC for 3 yrs or so, using a BT (UK) router
as my gateway.
Suddenly whenever I boot the PC it no longer sees the gateway and ping
doesn't work - doesn't even see itself any more.
I have found that the only workaround is to use the Network Setup Wizard to
create a new connection, which shows up as a Bridged Connection. If I then
delete that, the ordinary LAN connection works absolutely fine! (Until I shut
down, in which case I have to go through the same procedure). Why isn't the
normal LAN connection extablishing itself properly any more?
BTW I have two laptops both of which are still talking to the router fine,
so it doesn't seem to be an issue with the router (and anyway I have
power-cycled that several times to no avail).
Thanks for any advice!
CW