I'd go for a separate box because I've seen hacks for ESXI and other VM's so because you ultimately have little control over what can read the memory or access the disks on your main machine through a pipe for example, you best bet is a standalone independent machine like your old dell.
Bear in mind no AV software can find all viruses, plus the very nature of virus definitions is its just a list of whats been found and the vendor has decided is a virus. Virus definitions updates are an automatic process where software looks for "signatures" ie just a unique set of hex inside the files and decides what variant it is, when they find a new variation of it, they update their list and punt it out.
The actual task of deciding if a program is a virus can take many months of reverse engineering depending on how the programmer(s) wrote the original code, so just like it took over a year before anyone discovered and considered stuxnet a virus, so the same can happen today, ie you could get infected and not know about it for months.
It also never ceases to amaze me when I plug in old hard drives to retrieve something that a new virus is often found on the drive even though it might not have been used for over a year and was not found at the time of it being in daily use.
