Currently owned operating systems (most are installed on one machine or another, or in VMs).
DOS (several versions from 3.0 to 6.22, not all MS)
Windows 3.1 3.11 and Windows for Workgroups 3.11
Windows 95 & Windows 95c
Windows NT 4 SP1 (have the other SPs for NT4, just not installed)
Windows Millenium Edition
Windows XP Home SP1 & 2, and Pro, original build no SP)
Windows Vista Ultimate 32-bit (no Service Packs yet), and Vista RC1
Windows 7 Ultimate SP1 32-bit and all official betas
Windows 7 Ultimate 'N' SP1 edition 32-bit
Windows 7 Home Premium SP1 64-bit
Windows 8 Ultimate (Pro 64-bit/32-bit + WMC) and all official betas
Plus several non-MS operating systems as well....
GEOS for the C64
Linux (Puppy, Lindows, XandrOS, Ubuntu 6, Mandrake, Ubuntu 12, Mint 14)
I'm a bit of an OS collector/nut, as those who know me from EightForums can verify.
Started computing on a Casio PB100 (with 512 BYTES of RAM, expandable to 2kb!!)
which I still own, in 1984 (yes, it still goes, runs on two CMOS batteries).
Never looked back...
**EDIT; - @Doc Brown...Yes, I still have mine. 2x1541 drives (one for the OS which
was/is GEOS, one for programs) two C2Ns, and a 'Freeze Machine' cartridge (what a
wonderful accessory THEY were, though not cheap here in Aus). Still runs like new.
Can't kill a C64...
Also still have my 1MB A500....**