Another PC Mind Boggling Problem!

Every day at work I used to experience unusual problem, just yesterday I encountered such a problem. The unit is a P4 CPU, the only problem of the unit is it cannot connect the net, it always indicate a limited connectivity. I tried all the basics to fix the problem, but still does'nt work.

The network device was tested good in other unit,And I suspect that it maybe a software problem so I decided to install a different harddrive with a fresh installed OS, to check if it can connect to the net with no problem and it did. With that I decided to replace the OS to restore its internet connectivity. I install back its original harddrive and started the system, but I decided to check its internet connection again before fresh installing the whole drive and to my surprise the connection was restored.

It may leave so many questions in me, but I was happy though that network connection was restored with less effort.