There are programs that will securely delete files, and wipe free space, but as Elvandil has pointed out, there is info in other files that you might not think of, plus saved passwords in the registry for websites, email, network shares, etc.
That can all be cleaned out, but would be a time consuming process, and easy to miss something. Plus, hard drives write data at the sector level (512 bytes), but the OS file system writes in clusters (8 sectors, 4096 bytes usually). If a file with personal data had been deleted, and overwritten with another file that is a bit smaller, there could be a up to 7 sectors (3584 bytes) of recoverable info at the end of the file. Best to wipe the drives.
You can use almost any disk to re-install the OS, so if you can get a hold of a copy of the OS disks, you can re-install using the key for each PC. If you don't have the key written down, you can use a key-finder like
Magical Jellybean Keyfinder to find it before wiping the drive.
Only caveat is for XP (and maybe Win2K) the disk type must match the key type. If it's an OEM key, you can't use a Retail disk and vice versa.
HTH
Jerry