Problem with x64 but not on x86

I have a network with 3 machines 2 are XP Pro x64 and one a XP Home edition
x86, all had been working well for 9 months, until about a month ago, then
one day I could not reach Google on the x64s but I could on the x86.

Some other websites where not reachable on the x64s but all were reachable
on the x86. Some sites were reachable like the BBC but once on the home page
It would not let the X64s search but it would let the x86.

I looked in the event manager but couldn't find any errors for IE7 and none
that corresponded to that time frame.

It most certainly seems like a x64 problem as the x86 is not affected in any
way. no new software has been installed and they all use the same Avast AV.

Updates are installed by automatic update so I don't know if any of the
recent ones have caused the problem.

any help would be most appreciated.
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NS