On the same day Microsoft loudly proclaims Windows 8 in New York, the aging-but-still-going Windows XP today quietly celebrated its 11th birthday.

On Oct. 25, 2001, Microsoft launched Windows XP, unknowingly unleashing its most successful operating system ever.

If they only could do the same today, the company’s executives must think as they assemble for a day-long Windows 8 launch party.

“It was a good operating system,” said David Johnson, an analyst with Forrester, in an interview today. “It was a very, very good operating system … a superb OS because it removed a lot of pain.”

So superb, in fact, that it continues to run on an enormous number of PCs across the globe. According to Web metrics firm Net Applications, Windows XP powered about 41 percent of all personal computers—45 percent of those running one form or another of Windows—in September.

Only the much newer Windows 7 has a bigger share, and that only recently: It wasn’t until this August that Windows 7 passed XP to take the top spot.

By Forrester’s count, said Johnson, 48 percent of enterprise PCs now run Windows 7. But 38 percent of their systems continue to rely on Windows XP.