Malke wrote...

It sounds like you must have a combination modem/router. Some ISPs have a
limit on the number of computers you can use with their modem/router. Or
the modem/router may be faulty. Call your ISP.

I tried two other modems. Same thing happens.

And no, there is no limit on the number of PCs that I can connect. Besides, how
would it know?

Anyway, if that was the problem then the PC would not be able to connect or that
others would not be able to connect if such a limit was in force.

No, the problem is centred soley on this particular computer.

Today I made up a bootable Linux CD. I'll reconfigure the computer to boot off
the CD and I'll see if the problem still occurs when it starts up and loads
Linux.

At least then if the problem doesn't occur then it's definately a Windows issue
and not a hardware one.