Can Someone Access my Computer?

In General: No.

Windows Dial-Up Networking only allows your PC to access the Internet, and not the other way around.

Unless you are running some form of Server Software, that you have installed,

(eg: Microsoft's Personal Web Server, or an FTP Server), then no-one from the outside world can access files on your PC.

Windows does not come with any server software, so you would have to manually download and install server software to allow direct access to your PC.