There are several options.
Option 1 - Find someone who can boot the system and wipe the drive using the format command. Maybe even going as far as to tear down the partition tables as well.
The problem is that the data could still be recovered if someone is knowledgeable enough to do it. I've done it all the time with customers who have accidentally wiped their own hard drives or have had a virus do it for them
Option 2 - Take the drive out of the old PC and install it on another computer and run a wipe program that writes 1's and 0's over every bit of the hard drive. This is very secure as the overwriting of the data multiple times will make it harder and harder to recover with each pass.
Cons - Takes @#$%ing forever...especially with larger drives.
Option - 3 Remove drive and install it on another computer either as a second, internal drive or using an external chassis and using it as a data drive. This means that the drive is somewhere that *you* know where it is.
Cons - Age of the drive may mean that it may not be reliable for any given period of time. May last years, may conk out in a few weeks.
Option 4 - Take out the drive, put it onto a rock, grab a sledge-o-matic hammer and pound that sucker into tiny bits. Safe, secure, and cathartic.
Cons - make sure you have safety glasses on and long sleeve shirts and pants as flying bits can fly everywhere. Also can get messy when the platters shatter.