The main problem: out of two PCs, the old on e (WinXP) lost its Internet connection, as it appears to me, after I uninstalled COMODO Internet Security. I System Restored the PC to an earlier date and COMODO and Internet connections were restored. Yet, same day later "page cannot be found" popped up again. It is NOT a modem, a router or cable problem: the other PC (Win7) is Okay. The old one has no Internet when connected to the same router or directly to the modem. I reset (I hope, correctly) the Network (no Network now, I exchange files, etc. between the PCs though external HD), released and renewed the IP address and even used Windows repair feature - it informs me that "Windows repaired the problem, please try again".
The latter is part of the minor problem: Windows "think" that there is no problem: it "repaired whatever successfully, clicking the Systray connectivity icon (which always displays 100MB/s speed) System shows that the PC sends packets (although very-very slow!) and even receives packets, even slower. So slow that no page can be downloaded.
Checking for infections with Malware Bytes, S&D, Spyware Blaster was negative, COMODO informs me that there are no threats; I cannot run a scan as it asks me to "select an item (no items are displayed in the blank window!) to launch a scan".
What else can I do? I even replaced the connection cable, more than once! And the other PC has no problems whatsoever, with any cable.