please help with losing net connection and page not found - thank you very much

Hi, I have 2 very strange (irritating) problems and would really appreciate if you could help me out.

1 (major): I have a 24/7 internet connection, but lately, if I leave my computer alone for few hours I no longer can access the internet and have to restart the computer to again get my internet connection. Very rarely, I can Alt-Ctrl-Del End Task the IEXPLORE and it would work.
a) is this my ISP's fault?
b) something I can do? please help

2 (minor): Again started happening at same time as above. I suppose a consequence. There is a java site which I use and now when I go there the java part of the website will not load and will say "page not found". If, however, I click Ctrl-O and launch the site from this window, then everything works. Any ideas why it no longer works normally? (Ignore if its cause of the first question)

Note: I have zonealarm firewall, computer protected with norton antivirus, checked with trend micro online free antivirus, +ad-aware, +CCleaner, +(old) SpySweaper, +Spybot Search & Destroy, +HijackThis 1.97.7. Still doesn't mean I don't have adware. I have a feeling I have something which makes my browser show "page not found" sometimes.

THANKS A LOT IN ADVANCE FOR ALL THE HELP!!!