Microsoft accused open source vendor Red Hat of having "nobody except itself" to blame for the poor performance of the Linux computer operating system.
It claimed Red Hat had spent little money on R&D and therefore could not blame Microsoft and its alleged anti-competitive behaviour.
The software vendor's legal team then turned on Gateway's group counsel Anthony Fama, trying to portray him as biased and ill-informed.
Gateway, the beleaguered direct PC vendor which was forced to pull out of Europe last year, has joined those pushing for harsher penalties against Microsoft.
It claimed Microsoft still had far too much power over computer manufacturers.
But Microsoft lawyers tried to persuade the court that Gateway was an unfit witness because AOL Time Warner holds a large chunk of the PC maker's stock.
Gateway, AOL and its Netscape subsidiary, Sun Microsystems and Red Hat have put their weight behind the states in their anti-trust battle.