Tech support is stumped? Didn't they have you download a new driver for the Network Controller and install it? The yellow exclamation point in Device Manager is indicative of an error or issue with that particular hardware component. The error message stating "Drivers for this device not installed (code 28) backs up and clarifies that drivers are needed. Does it really matter, at this point, how the driver(s) came missing, or shouldn't they be concerned with helping you locate the appropriate driver(s) either on your Drivers and Utilities disk which should have come with your system, or download them from the Network Controller's manufacturers website, or from your system manufacturers website?
Anyway, that is exactly what you need to do. Who is your system manufacturer? Each manufacturer has a Drivers and Downloads page on their support website for each particular system they build. Dell has this, HP has this etc. If, for any reason, can not locate the appropriate driver from your manufacturer, you can still either use your Drivers and Utilities disk, which is sent with each purchased system, (after all, needing to reinstall drivers and the special utilities of each manufacturer is common) you could attempt to get the necessary drivers from the Network Controller's manufacturers website.
Try this: browse to your system manufacturer. Click on the link for Support. If necessary, click on the link for Drivers and Downloads. A page should come up asking you to identify your system. Enter your model and serial number, or Service Tag. If you have a Dell your Service Tag will be on the All Programs menu in the Dell folder. Write that down. If you have a Dimension, type that and its number, such as Dimension 5150. If needed, enter the Service Tag. A page will now appear with categories of all the devices, components and software that came with your system.
For the Network Controller, click on Network, or whatever title used for that category. Make sure you write down name (if possible) of your Controller, and its serial number. If needed, open Device Manager, double click on the Controller name to open Properties, click on Details, then on Hardware IDs. The name and serial number might be there. Or, type this into the Run box: msinfo32 and hit Enter. A new dialog box will appear.
Click on Components, then on Network. Scroll down the list until you locate the name of your Controller. Look next to PNP Device ID. You can identify your serial number there after the _DEV and before the &.
Now, click on the appropriate device item, and print out the installation information. Download the driver(s) to your desktop (XP) or your Downloads folder (vista). You can download to the desktop in Vista if you like and think it will be easier for you to locate during installation.
Follow the installation instructions on the download page.
If for some odd reason you cannot locate that information at your system manufacturer, try to locate your Drivers and Utilities disk that came with your system. Pop it into the CD/DVD drive, and instead of running it, open it to examine the folders on it. Look in every folder if you must, but locate the name of your Network Controller. Make sure you find the correct one for either a 32 bit or 64 bit system. Do you know which type of OS you are running? If not, look in System Information again, only this time click on the top option: System Summary. Look down the list until you locate the entry for System Type. If it says X86 it is a 32 bit OS. If it says X64 it is a 64 bit OS. Make sure you download only drivers or other items which is for the correct OS type you are running on your system.
Now, after you locate the driver, drag and drop it to your desktop. The proper permissions should automatically be written to the file.
Open Device Manager, double click on your Network Controller, click on Driver, click on Update Driver, and then follow the instructions on the dialog box. When you get to the place where it asks where to look, click the browse option, browse to your desktop, click on the file to load it in the window, and click Install.
If you cannot locate the correct driver or do not have the appropriate disk, go to the manufacturer of the Network Controller. Mine is Marvel Yukon. Go to whatever the manufacturer is, and search for your particular model or make, and click for Drivers and Downloads. Depending upon how the Support section is outfitted, you may just need to select it from a drop down list, or an action box window, or click on a image.
Download to your desktop or Downloads folder, and follow the same steps as above for installation.
Good luck and much success getting your drivers and your network connectivity back up and running correctly. Please have a nice day.