Just a similar name.
The Year 2000 became abbreviated as Y2K (as BC has explained, with the K for kilo). The abbreviation got a lot of attention as the start of the new millenium, which was technically incorrect -- the year 2001 was the start of the new millenium, but ninety percent of people were like, "Whatever..."
Then the whole mess with all the computer systems which were not going to be able to handle the date change. It was kind of like the people above -- most computer systems were like, "Whatever..." Most computers didn't care.
Windows 2000 simply became known as Win2K (three syllables) -- not in any direct connection with Y2K, because Win2K, as with other versions of Windows and the hardware running it, could handle the date change. It was the older computer systems -- legacy systems -- mainframe systems -- computers long controlling power grids, railroads, etc. which were thought to be the danger.
