How old? What CPU does it have? Yes, you can run stuff like VirtualBox on older systems without the CPU virtualization support newer CPUs offer, but the VMs will run slow and somewhat less reliably. The main thing is the number of cores the CPU has and even more importantly, how much memory. Also, remember that you cannot run 64-bit virtual machines on a 32-bit CPU. At least not that I would want to... :-)
The question should be: What do you want to do/achieve inside those VMs?
I recall DOSEMU that is also a VM, but just DOS type 16bit real mode stuff. VMWare, Virtualbox, qemu will run on just about anything 386 or better, however as mentioned before, it won't be that "fast" as you won't have hardware assistance of the later VT-x/vmx/SVM/AMD-V instructions, as is described in http://en.wikipedia.org/wiki/X86_virtualization