Malewarabytes has a free version.  I don't remember if there is a separate download the for the free version.  If your old drive is plugged in, try to launch Malewarabytes.  If it launches, run the update for it and then run the scan a couple of times.

If it won't launch, still consider buying it.  Even if you have a good AV suite, I found that Malwarabytes tended to pick up things the AV missed.