Should I completely reformate and sanitize my hard drive?

My dorm room was broken into the other day and in the strangest twist of things, my iPad and a USB stick was stolen, but the perpetrator left my laptop right where it was! (And it'a fairly new one, only 7 months old)

It's had me scratching my head for awhile, why would someone break in and steal one thing but leave something else behind? Then I considered the fact that maybe he could have briefly switched the laptop on, plugged in usb stick with a keylogger that automically copied itself to the hard drive, and then left it? I use a standard windows password so I'm not sure if it's possible to still do that at the password screen without entering the password… Should i just wipe out the hard drive? This has me paranoid as hell--it's like stealing a Rolex out of a Ferrari with the keys in the ignition and leaving the Ferrari there!

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