How to allow someone to access my computer with their own username

Hi,

I'm using a laptop, running windows vista. A friend and I often share files
between us and when connecting to my computer via network he uses my username
and password to connect and take the files I have shared.

Ideally I would like him to click on my computer in the network then
automatically view my files under his own account name, or at least be able
to type in his own username and password. BUT I do not want to have another
account active on my own computer. (ie, 2 user accounts available on my log
in screen).

Also, I obviously do not want any security flaws so only he can access my
computer with his details.

Many thanks in advance,

Smithy963.