I have yet to see an ethernet card incompatibility problem in my life. Ethernet technology has been around for over 20 years, so it's highly stable and understood at this point.
The one thing you don't mention having tried, is scanning for viruses and spyware on your mum's system. Since she uses IE, there's pretty much guaranteed to be some, it's just how much. Spyware is an ever growing threat, and IE is the primary means by which it gets onto a system, so a word to the wise is to migrate to something a little safer like Mozilla Firefox.
Anyway, why don't you describe what precautions you take to prevent viruses and clean up the inevitable spyware infestions that come with regular IE use.
my moms computer has had a slow connection since cox gave her a new terayon tj715x modem. its installed correctly, ive done pretty much everything i can think of (scandisk, defrag, recovery disk, new cat-5 cable, manually configured tcp/ip's, reconnected modem, called cox tech-they didnt know much-suggested same things i had already done.) im wondering if the new modem is incompatible with the ethernet card that is installed. I dont know what brand it is but it is old(it was installed new in late 98, early 99 timeframe). The computer is also old but still works good.500 mhz processor 224ram, o.s is windows 98. internet browser is updated to i.e 6 sp1. i am pretty much lost. any help would be great. thanks