It depends to what degree of security you need.
- Terrible: Delete all of your files. (The bad guy can still pull them from the recycle bin!)
- Minimal: Delete all of the files and empty the recycle bin. (There are ‘undelete’ tools that can still get the files back.)
- Better: Use a tool like “Eraser” which writes zero bytes over unused sectors of the hard drive. (This is fairly good - providing you deleted ALL of your files - including stuff in temp folders and in places that individual programs might hide them.)
- Better still: Reformat the hard drive - fully (not a “quick” reformat), then reinstall the operating system from the original media. (This is MUCH better, and probably good enough for 99.99% of people…but it’s not perfect. Hard drive heads wobble a bit as the disk spins under them - which can leave a tiny amount of the original data track still present. People with deep pockets like the NSA are sometimes able to recover data still.)
- Better yet: Do that many times over - when the computer is just turned on - and again when it’s been running for a while and everything inside is warm. (This is even better because at different temperatures, the head will take different paths over the magnetic tracks…but it’s still not utterly perfect.)
- Still better: Remove the hard drive and smash it to pieces with a sledge hammer. (But people with deep pockets can read back at least some data from broken and scratched disk platters)
- Even better: Hold a large magnet over the drive while it’s spinning and THEN smash it to pieces with a sledge hammer. (A magnet should totally erase all magnetic signature from the drive - and a sledge hammer helps some more!)
- Best of all: Find someone with a blast furnace…drop the hard drive in there! (DONE!)
But honestly - unless you have military grade secrets - or you’re a very high ranking drug lord…reformatting the hard drive is the safest and sanest thing to do. Regular criminals and low-grade hackers can’t recover data from that.