Orka was exactly right though. You did not provide us with nearly enough relevant information, such as:
Which version of Windows? XP, Vista, 7?
Do you use a wireless connection or a wired one?
Which service pack were you installing?
Does your computer recognize the network and fail to connect, or does it fail to see it at all?
Do you use Windows's built-in firewall or that of a third-party antivirus program?
Do you allow Windows to configure your connection, or do you use a third-party utility?
It was most likely a compatibility issue between the network adapter's driver and that service pack. Had the newer service pack not fixed the problem, you might simply have needed to update the driver for the network adapter.
FYI, it is not at all unusual for a large computer corporation not to offer free over-the-phone tech support for products out of warranty (and 2 years is a year outside most PC manufacturer warranties). Among many other reasons, this is because an enormous number of problems or issues (such as the one you were having) have virtually nothing to do with Dell and everything to do with Microsoft.