Windows XP till this machine dies a natural death.
A recent hard drive reformat, new modem, cleaning of the keyboard and tower case, and only just the programs I need on it, has my system running great. Since all I do is E-Mail, Web Browsing, and Word Processing, upgrading to Windows 7 would probably not have me doing new things. In short, this computer does everything that I need it to do, and for it's age (8 years) works just fine.
I have a relative who is a computer engineer and we will just start swapping out hardrives, OS updates and other upgrades when the time is needed to do that.
Jack