My iTunes will not open and Flash will not work

When I try to open iTunes, it just bounces at me and does nothing else

When I try to open a webpage containing a video or Flash, either the webpage hangs (and I get an Apple error screen) or the flash player fails

This appears to have happened after I tried to install a Flash update, but which is chicken and which is egg I don't know

Oh - secondary question - which may be related

I have a VERY slow and unreliable broadband connection, and whilst I can often get a PC tablet to connect, sometimes the Mac will refuse to complete webpage uploads - this also sometimes causes problems accessing Apple support and updates. WHY does this happen, CAN this cause a corrupt installation, and HOW do I get the Mac to be as good at accessing a poor broadband service as a PC? I also have an Aiport but this refuses to see any internet connection at all.