The drivers for the ethernet adapter can be downloaded from:

http://support.dell.com/support/downloads/type.aspx?c=us&cs=19&l=en&s=dhs&SystemID=INS_PNT_P4_5150&os=WW1&osl=en&deviceid=2966

If your problem is occurring because you have just reinstalled Windows, you need to make sure that the chipset drivers are the first drivers installed.  The chipset drivers can be downloaded from:

http://support.dell.com/support/downloads/download.aspx?c=us&cs=19&l=en&s=dhs&releaseid=R58370&SystemID=INS_PNT_P4_5150&os=WW1&osl=en&deviceid=1134&devlib=0&typecnt=1&vercnt=1&formatcnt=1&fileid=70519

Stevev