How old is this old PC? The guys above me are right - you can't make an "Arduino" out of one, but if it's of a certain age (we're talking early/mid 80s and green/black screens here!), you may still be able to salvage some useful bits out of it.
On VERY old kit, you sometimes see MCS-48/51 family microcontrollers used for keyboard scanning processors and other low-level pin twiddling tasks. These may be useful, but are also a pain in the arse to harvest and do something with. If they're not socketed it can be quite hard to de-solder a 40 pin DIP and extract it without damage.
Also, they are probably mask-programmed with no provision for you to burn code directly on to them, and far less than 1kB of SRAM. So, you would need to set them up to execute instructions from external memory, and provide this memory for them. An EEPROM to hold your program code, and some SRAM, and more than likely some I/O expansion chips depending on what you want to do. Then the address decoding logic... and either learning some assembly language for them or finding a suitable C compiler. It's certainly do-able, you'd end up with something a little bit like this
But, in the time it took you to do all that, you could have gone to work and earned enough to buy several nice genuine Arduino boards