Well I actually managed to fix the problem so I thought I'd share it in case others out there had something similar.  Apologies to those reading for the step by step instructions. The Latest Broadcom driver from Microsoft via windows update fixed my wireless problem. No surpirses there. The wired network card is fixed by the following, (step by step instructions given in case a newbie techie reads this): I've no idea why this should be a problem as auto negotiate should just pick the right settings itself. No doubt in a few weeks or so there will be an updated driver via Microsoft update or Dell but for now at least the problem is fixed.
  I've recently installed windows 7 64bit Home Premium onto my Inspiron 530s.  A fresh install into a new hard drive.  Over the last few days I've had intermittent problems with the wireless and Ethernet cards with very slow performance from either card.  I've installed the latest BIOS from Dell and attempted to download and install the Vista 64 drivers.  On installing the Vista 64 drivers the installation failed because it said I didn't have an Intel network card installed.  Luckily enough I still have Vista installed and the network and wireless cards work just fine. Unfortunately Dell haven't provided any Windows & drivers so I guess its early days for a fix.  I'm wondering if anyone else has had this problem and if so how they fixed it.  According to the devices list I have  Intel (r) 82562V-2 10/100 & Broadcom 802.11g (wireless) network adaptor. All help much appreciated, I've spent the last 3 days trying to fix this and having to switch back and forth between Vista and Win7 is doing my head in. 
Windows 7 - Inspiron 530s network card performance issue