i'm with embro. although i'll admit that i've snooped used computers before, it's usually to find cool software that they installed. you'll get the coolest programs sometimes, but it's actually illegal for them to selll you the comp. without the liscenses for it.

from a security standpoint, you don't know what they downloaded before they sold it to you, so they may have (intentionally or unintentionally) installed a trojan on their computer before selling it to you.

tada! you've now been hacked by some 1337 script kiddie. look out!

but anyway, it's up to you. my vote: reformat