I have a old desktop computer that I want to make a home server for file sharing purposes. It has 512 MB RAM and few GB of hard drive. Which operating system should be reasonable for this kind of slow system.
Only a few Gigs?
A linux distro would work I guess but adding even a 40 gig in there would greatly expand your options.
I run a web/file server on a PIII 450 with 640mb of ram. For a 'pure' fileserver, freenas is nice, else going for a cli only install of ubuntu works well. Hardwarewise, you'd need at least one network port, naturally, and you could set up the box headless, doing admin tasks via openssh. I'm using less than 100 mb of ram right now with a file/web server based off ubuntu so... yeah, it would probably work
An OS such as Minix would fit, but it's not clear if you can do any more than learn about OS internals.
This is what Minix was designed for.