This problem accured twice to me,
After I downloaded and installed beta SP1 for vista, 32bit programs (firefox, msn messenger, IE7 32bit) couldnt access the internet (and even crashed oftenly)
althought 64bit programs could access the internet normally (IE7 64bit), so I just reformatted and reinstalled vista.
Now it happend again, this time without any apparent reason,
It happend tonight after I ran windows update (it installed 2 updates: Synaptics PS/2 Port TouchPd, HP Quick Launch Buttons 64)
and now the issue is back again.
Again, it's very important you understand that this issue happens only when I run 32bit programs but when I run native 64bit programs everything works fine.
I think it's a problem with the 32bit emulator but I have no idea how to fix it.