What are you guys doing to clear out the data in your old PCs?
I deleted my files, and then used cipher (available in Windows 7 and later) to securely delete.
A quick Google search will get you pointed in the right direction. Basically, it can be used to write over unused disk areas with random data to securely delete it. When you 'delete' a file, it's really still there on the hard drive. The OS just sets a flag that more or less tells it that space is now free. However, the actual data is still there, and could be recovered with the right tools. Cipher basically writes over the unused space 3 times: once with all 0s, once with all 1s, and then once with random values.
For my older Vista laptop, I wiped the hard drive with DBAN and then re-installed windows. I would have preferred to use DBAN/reinstall on both machines, but I no longer had my windows 7 installation disc, so used the cipher approach on that one.