Is this the driver you are using at the moment?
http://global-download.acer.com/GDFiles/Driver/Lan/Lan_Realtek_5.722.416.2009_XPx86_A.zip?acerid=633929958957005746&Step1=Netbook&Step2=Aspire One&Step3=AO751h&OS=X02&LC=en&BC=Acer&SC=EMEA_1
Have you installed the other chipset drivers already?