How was my friend able to do this on his computer? o.O?

Okay so today I went to my friends house and I took my desktop computer over there and when I connected my desktop to their router (he has an ethernet port in his room) for some reason he was able to go through my files on my computer, which I found really weird... no hacking program was used either... why or how was he able to do this? he was using a laptop, wireless and we both have Windows 7 Home Premium 64-BIT lol. Thanks for any info! ^^