The problem is : I can't go to www.avast.com. My other PC can, and I used to register for the second time cause I had reformated my PC. Is there anything that prevent from visiting Avast web site?
Yes, your browser security settings restricted sites, your firewall, a virus possibly blocking access to anti-virus sites to stop you getting rid of it, an add blocker, etc. There really are to many permutations and there isn't any information to help.
What is your OS, is it fully up to date ?
What is your browser, have you tried a different browser ?
Do you have a pop-up or Ad blocker, if so what ?
What errors or warnings do you get when you try to reach the web site ?
How are you trying to reach the site, using a favourites link, typing it into your browser, etc. ?
If you are using avast for Home non-commercial use and the computers are in the same house you can use the same registration key on more than one PC, so you shouldn't have to register again. However, that doesn't explain why you can't access avast.com, are there any differences between both systems ?