Windows XP can't find Yahoo.com...sometimes

[doubleplusgood]

I have 2 machines side-by-side in my office, both connected to the internet through the same router: one is an HP Windows XP Media Center PC; the other is an older IBM PC with Windows 98 SE.
I have http://www.yahoo.com set as the home page on both machines & use IE 6.0.2900 on the XP & IE 6.0.2800 on the 98SE.

Problem: Occaisionally, the XP machine can't find the Yahoo home page, giving the "Cannot find server or DNS Error" error message. At the same time, however, I CAN still get to Yahoo on the 98SE machine.

Any ideas on what may be causing this problem on XP ?

TIA