I need help and I hope there's someone here who can point me in the right direction

For the last few years, I have been regularly accessing several official sites, mainly www.uscis.gov
A couple of days ago, I started having problems.....at first I could still access the homepage, but when I tried to access further information on it, I would get the typical error message ("Internet explorer cannot display the website. Bla bla"). A day later, I could not even access the homepages anymore (uscis.gov, travel.state.gov, just to name a few). Since then, nothing has changed.
The weird thing is, that I can sometimes pull up the homepage ONCE after starting up the computer.....but when I close the page and try to open it again, I again receive the error message.
At first, I suspected a virus or spyware, but I have since tried it on all three of our computers (two laptops, one desktop), and it's the same on all three. No one else is having these problems (I asked several people), so it can't be a problem of the sites themselves.
I have Windows XP on the desktop and on one of the laptops, Vista on the other laptop. The laptops have a wireless connection through a linksys router. Our internet service provider is MTA.
I usually use Internet Explorer, but have tried it with Firefox as well, just to make sure (had the same problem, though).
Can anybody here help me.....??


I counsel immigrants (legal ones, from Germany

I would REALLY appreciate some support

Many thanks in advance,
Denali