Driverpacks are a collection of drivers for Windows.  See here  http://driverpacks.net/

I have used them and if you're installing from scratch on an unknown system they can be a time saver.

Now in this case, does the hard disk come from another HP Elitedesk or another brand?  You can't normally swap hard disks from say an IBM to a Dell without experiencing problems ranging from registration and chipset problems.  There are ways around this but that's a little beyond me.

Most common problem in ethernet drivers not installing is the chipset driver.  Once the chipset driver gets installed all of the other drivers can get installed and usually automatically.  You should be able to download the chipset drivers from the HP site.

There are several network adapter options available for the HP EliteDesk 800 35W G2 Desktop Mini PC.  If you know which one you have, download it directly from HP.  If you don't know which specific network adapter is in your computer, download all the drivers and try them one at a time.

https://support.hp.com/us-en/drivers/hp-elitedesk-800-35w-g2-desktop-mini-pc/7633266

i have already downloaded the drivers directly from the vendor's site: https://support.hp.com/us-en/drivers/hp-elitedesk-800-35w-g2-desktop-mini-pc/7633266

This includes the chipset as well as the network controllers but still with no luck...my last option is if no help then I will reinstall the OS and as I mentioned clearly, I hate to lose these licensed software currently installed on the machine as most of them are no longer available online and purchasing a newer license is a bit high.       

This model has both wired and wireless network adapters, yes?  Neither of them work?  Is it possible the network adapters are disabled in the BIOS?

Have you tried the network adapter drivers from the Intel site:
Intel® Ethernet Connection I219-LM

Intel® Dual Band Wireless-AC 8260

Are you using Windows 7 or 10?  

It probably would have been slightly easier to convert the old system to a VM and run it on the new OS rather than trying to shoehorn new drivers to it.

i will try these now:

https://www.intel.com/content/www/us/en/download/15084/intel-ethernet-adapter-complete-driver-pack.html

https://www.intel.com/content/www/us/en/download/777192/intel-wireless-wi-fi-drivers-for-intel-dual-band-wireless-ac-8260-and-intel-dual-band-wireless-ac-8265.html

Here is the device manager looks like (attached): 

still failed. 

Go back into Device Manager, delete that Ethernet Controller, and all the other Unknown Devices. Delete that Intel HD Graphics 530 as well. 

Reinstall the chipset driver using the download from the HP site. 

Then reinstall the graphics adapter driver using the download from the HP site. 

Then reinstall the network adapters using the downloads from the HP site. 

Then reboot and see if those devices show up. 

ok sounds great i shall do that if not then I guess we just have to reinstall OS. 

i will proceed as mentioned @paulmacdonald. 

i must say too that this SSD drive has a percentage health of 62% (HDD Sentinel check/test result). will reply here once I completed as per mentioned @paul

I have uninstalled as mentioned and I have install it but still same issue.

I will leave it as here - I believe the issue here encountered in bad sectors on the HDA (62%) which I believe it is causing this error to be very stubborn. 

what disk is it?  model please - find it in device manager>disk drives

you can check the system files with sfc /scannow from an elevated prompt

you can run Hddregenerator for repairing  bad sectors ( not free)

In Device Manager go to the Ethernet Controller with the error flag

Right-click on it choose Properties, then in the window that opens click on the Details tab

Change the drop down label to Hardware IDs

Copy the first string listed (right-click > Copy) and paste it here

Repeat with the first of the Unknown Devices below it

I've completed the manual installation process using the .inf file for my computer's driver, but unfortunately, it's still not functioning correctly.

After evaluating the situation, it appears that the problem may stem from bad sectors on the drive. Typically, chipset updates would address these issues, but it seems the Head Disk Assembly (HDA) is at fault in this case.

I've decided not to spend any more time trying to troubleshoot this issue. Despite numerous attempts and various methodologies, I've been unable to resolve the problem with the Ethernet driver.

Regarding the other unidentified drivers, they are all listed under the same name as shown in the provided screenshot.

Best regards.