Don't know how many people are still using Win98, but I think you should be aware that the NetworkOptimizer tool found on PlusNet's portal does not seem to work for Win98.
When I ran it, it only found one Network adapter, the DialUp adapter. So I proceeded anyway, but of course this made no difference to the DSL connection speed. I looked in the Windows registry to see what the tool had done and then manually applied the same to the DSL modem adapter (it had added a DWORD entry called "MaxMTU" set to 145.
This had a nasty effect on my performance, pings and traceroutes looked ok but speed tests and d/l were down by about 40%.
When I checked further on the internet I found the issue:- For Windows 98, the MaxMTU entry in the registry should be of type String not DWORD.
Hence even if the tool had found my DSL adapter it would have added the wrong type of value.