Seattle ? Microsoft Corp. said late Thursday that parts of its Windows source code — the tightly guarded blueprint of its dominant computer operating system — had been leaked over the Internet.

Microsoft spokesman Tom Pilla said that some incomplete portions of the Windows 2000 and Windows NT4 source code had been “illegally made available on the Internet.”

Access to the source code could allow hackers to exploit the operating system and attack machines running some versions of Windows. Several versions of the operating system, including the ones containing leaked code, are used on millions of computers worldwide.

Such access could also provide a competitive edge to Microsoft rivals, who would gain a better understanding of the inner workings of Microsoft’s technology.

The company was made aware of the leak Thursday and was investigating, Pilla said.