Win95
Microsoft Windows 95

Came out in about 1995.
Quite a change from Windows 3.1 which preceeded it.
A number of versions were releasted. The best is OSR2 (or even later, OSR2.5).
The later versions offered some USB support, but not very usable.
Can boot with 16MB of RAM, or even less.
Can be installed in about 200MB of hard drive if you are somewhat desperate.
Runs OK on a 486. (Might have problems booting if your CPU is too fast!)
Followed by Win98.