password changes and different words on 2 computers on same account.

I can't seem to get my Ubuntu pwds established so they authemticate a download or upgrade. I need to change pwd but how many does Ubuntu need except for options for special community sites, purchases, etc?

I have a 2d computer 12.10 Ubuntu unit which I use offline as a guest only. If I assign a pwd & username and go online with the same ISP account like my other machines will this cause a conflict?

So far I use it like a wiondows machine for internet only and don't use any Linux capabilities proper though I do have some Linux apps on one machine such as sound, pictures, etc. I did like the early versions with the flying toaster screen savers, etc. fsp.