Why? If it works and does the job, then what's the problem? Malware is typically acquired through user stupidity, not through an external attack on the OS.

I'm typing this on an XP Pro laptop... it still works fine and is running mostly non-MS software anyway (OpenOffice for editing, GIMP for image processing, Chromium and Firefox for web access, Thunderbird for email). If the HDD takes a dump, I'll probably reinstall with Win 7 Pro, but other than that it's not worth the time or effort. Certainly not going to Win 8/8.1 with its ***** interface and M$'s push for me to put my personal data in the "Cloud."

Backups happen to a Linux home server that in turn gets backed up to encrypted cloud storage regularly, so data loss is a non-issue.