Network drivers from Windows XP PC automatically uninstalled. What is the reason behind this issue?

I don't think it is possible that your network driver has got uninstalled automatically from your PC. Definetly, somebody or may be some bad programs would have done that. But not to worry, If you can see that you don't have Network Driver in your XP PC then you can easly re-install the driver.
Here is the the stepwise details, I hope it'll help you.
1. Right Click on Computer
2. Click on Manage
3. Look for Device Manager
4. Here you can see the Network Adaptors and the detail about that(may be you'll find some yellow mark over there). Right click on it.
5. Click on properties and look for the network adaptor manufacturer name. It'll be something like PCI or Ethernet Controller.
6. Google for the driver using the adaptor name for Windows XP, download it and install it.
7. Restart the system, It should work fine.

If network drivers from your Windows XP PC have automatically been uninstalled, there is only one reason behind it, that is:

There is a virus in your computer. So, if you want to solve this problem then you have to buy an antivirus program that'll remove viruses and malware from your PC.