Yes you can use an old computer to build an altcoin mining rig. Here are the important points to know when building a cryptocurrency mining rig using antique computer parts.
First of all you only need a 64 bit processor. This goes way back quite a few years. My old computers were running Windows XP when I got them. I use old Dell motherboards which are still working well and make reliable dual card mining rigs.
My old Dell motherboards came with dual core Intel Pentium processors. These are very old but still work well for mining.
When mining cryptocurrency the CPU is basically only there to boot up the computer and then run the operating system. After booting up, it does not do very much. The graphics cards then do all the work mining your altcoins.
Many people shut down the extra cores of their CPUs after installing the operating system in order to reduce power waste.
I would recommend running Windows 8 or 10 but you can get by with Windows 7. It is harder to get software though for anything under 10 so that is what I would advise.
I have run my rigs with as low as 1 GB of RAM with no problems. But I am using Nvidia graphics cards and not AMD. I cannot make any claims with AMD other than I have run them on 4 GB of RAM successfully. I just never tried to go lower with AMD.
Windows 10 will boot up with 1GB of RAM albeit slowly.
I would recommend at least 40 GB of hard drive space but 80 is better. More than that is just wasted space.
These old Dell computers have SATA HDD slots and they work well enough. I have also used Solid State hard drives with success but on newer and faster computers.
There you have it. Please feel free to ask my any questions below in the comments.