Cannot install/update Broadcom 57xx Ethernet driver on Optiplex 740

I acquired my Optiplex 740 in April 2007 - Spec as follows:

OptiPlex 740 MT - AMD X2 5000
2.0GB 667MHz NON-ECC DDRII Memory(4*512MB)
160GB (7,200rpm) SATA 3.0Gb/s Hard Drive
ATI Radeon X1300 Pro PCIe 256MB, DVI (Full Height)
13 in 1 Media Card Reader
Windows XP Professional SP2

This week, I downloaded the updated driver for the Broadcom 57XX Gigabit Integrated Controller from the Dell website (R161008, released 17/10/2007). When I ran the download, the files extracted to the dell/drivers directory, and the installation program started. At the end of the installation, a dialog box popped up stating that "The installation could not be completed. Please attempt to run the installation again later" (or something very similar - I'm not in front of the PC whilst writing this, so cannot replicate the error).

From that point on, my network card stopped working and I have no access to the internet or the other PC's on my network. I have tried:

* Rolling the driver back (no driver could be found)

* Re-installing the original driver from the Drivers/Programs CD that came with my Optiplex (produced same error as the updated driver - installation could not be completed)

* Installing the drivers using the Add Hardware applet in Control Panel (it can recognise the driver as correct for the hardware, but I get an 'Access Denied' error when trying to install)

* Deleting the network card from Device manager and rebooting (it sees the card upon reboot, but says there is a problem when trying to install the driver).


I've tried everything I could think of, and Google turned up no answers. Please help - it's driving me mad! I cannot currently be in front of the offending PC at the times the support desk is open, and need to get this PC back on my network.