We wiped off everything before we disconnected it......but you never know....
If you did this within the operating system - ie using Delete - then all you have done is delete the entries in the table that record the physical location of the data. The actual data is still on the disk and can be recovered.
The best way to dispose of an old PC is to remove the hard drive(s); leave the rest at the local recycling centre; and destroy the hard drive(s) as discussed above. You used to be able to disassemble a hard drive using just a Philips screwdriver; but now they seem to use specialist fasteners and/or glues.
If you intend to use a drill you should try and find someone with a pillar drill stand for stability. Do not forget eye protection.
Ken