Hmm.... Not quite sure what's causing this, but let's see if we can figure it out.

For starters, why is that IE 5.5? You should be using 6...

Also, what are the specs of both machines? The specific operating systems, in particular (was it 9.2 on the Mac with problems?).

When did this start happening? The only major Hotmail and Yahoo updates I've noticed recently were the increase in storage capacity, and that didn't even happen on my MSN account...

Are you running the latest version of Java? At this point, the Java initially included with IE (illegally, might I add) will simply not work with many sites and you need to install Java from Sun Microsystems.

What kind of security software (spyware removal/prevention, antivirus, firewall) are you running, both on the Windows and the Mac?

Is the windows maximized and the computer at 1024x768 when this occurs? If not, try it, although I'd say you probably have had it at 1024x768 while maximized at this point, based on what your second post said.

Have you tried the latest version of Mozilla Firefox (should have specified Firefox)? It is Netscape-based, but I've no problems like this with it in Windows 98, 98SE, 2000, or XP.

Can't tell you much about Macs, but the fact that this occurs on both a Mac and a Windows machine is likely some software you either are running or aren't running and need to be.