If you mean a current windows machine

open a command window (start -> run -> command) and type

ipconfig /flushdns

As "outsider" said, you can use ipconfig /flushdns.  In addition, are you
using XP or Vista?  With either one (and while in Internet Explorer), I
*frequently* go to Tools>Internet Options and then delete files, history,
and settings.  That can be done in one step with Vista and individually in
XP.

The reason I askeds if you are using Vista is that Yahoo may cause a problem
there.  I bought a new computer in January, and for awhile I got frequent
"Internet has stopped working" error messages.  This would happen several
times a day, and always while I was in IE.  It would frequently happen when
I clicked on an ordinary link (such as those in CNN.com).  On other
occasions, IE would suddenly either freeze or close and automatically
restart.  I finally called Microsoft support, and the woman I talked to was
very helpful.  One thing she had me do was go into Control Panel and delete
any references to Google or Yahoo (but Yahoo seemed to be the culprit in my
case).  She said there are sometimes (not "always") incompatibility problems
with those two search tools and Vista, and she recommended that I use the
search bar at the top of the screen that is the default, or go to a search
tool manually.  This has worked very well for me, and I have not had a
single error problem since that time.  Unfortunately, I don't remember the
other step she had me take, but I think she had me make some change to the
add-ons.

MaryL

If after trying the other steps suggested you still have a problem, try
resetting your modem. You will find a reset button on the back panel where
the cords connect. You'll have to use the tip of a pen or something to push
it in. Then you will have to re-create your internet connection per your
ISP's instructions.

Jo-
You haven't given enough details, so here are some generalities;
There are two *main* reasons for your symptoms.  One is a lousy connection, the
other is just a "plugged up" router.  You obviously have the solution for the
second one.  For a bad connection, all you can do is reset the modem/router.  Of
course, this also depends on the modem, so you'll have to read your manual
carefully.  If you aren't sure how to reset, just unplug *from the wall, not the
back of the unit* for 10 seconds or so.  You shouldn't have to redo any settings
this way.

If it only happens at some sites, it may just be a lousy site plugging
everything up.  Yahoo has always been bloated so I avoid it whenever possible.

If resetting the modem fixes it for awhile, but the problem returns, then you
most likely have bad line conditions.

These are generalities only...  !!!