Mainly monopoly laws, or anti-trust as the Americans call it. Also unpleasent buisness practices.
In no paticular order it was illegaly nasty to IBM.
http://www.theregister.co.uk/2005/07/01/ibm_ms_settle/
There was also deliberatly writing programs to break if IBMs Dr DOS was detected so programmes would only work on MS DOS. Not perhaps illegal, but underhand.
Commited serious anti-trust offences (Became a monopoly illegaly and then abused its position)
http://www.theregister.co.uk/2002/11...eal_microsoft/
But got off when Bush got into power (allegedly
). In any case the DoJ cut a deal shortly after the election
Oh and annoyed the EU by abusing its dominant OS position to force feed media player to customers. There are other cases still ambling through the courts.
http://www.theregister.co.uk/2005/06/09/ms_wmp_windows/
There has also been a very clear understanding, if OEMs and system builders wanted to have Windows then on competing software could be bundled. This would be fine, but by this time ('98 ish) Windows was a defacto monopoly and so it would be commercial suicide not to sell PCs with Windows on. This is called 'Abusing a monopoly position.'