Hard Disk Drive Full But Shouldn't Be

I went to a computer shop today run by a very honest man. I asked him to quote a price for upgrading my mother-in-law's laptop with a larger hard drive. He asked me if I had any external storage. I told him yes I had a 500 Gb external hard drive. He said take everything you want off the laptop onto that. Then use the recovery disk to re-set the laptop to the factory settings and you will acquire a lot of space. So, a completely free way to solve the issue. Another place wanted UK£50 before they even looked at it. Nice to know there's some really honest folk out there if you look.