What OS were you using, and how did you reinstall it? was it a clean install or simply a recovery on top of existing OS ?

Also, try getting another computer or laptop and connect to your modem to see if the speed is ok. This will verify if your computer is to fault...

Also, you should look at your Realtek LAN card, swap it into a good computer or put another NIC in your computer.

Any more info would be good?