Right click on file in explorer, then "TortoiseSVN", "Show log".
Uncheck "Hide unrelated changed paths"
Uncheck "Stop on copy/rename"
Check "Include merged revisions"
Click "Show All"
Now, in the top pane you a list of revisions with username, date and log. Click on a revision you're interested in and you can see the full log in the second pane. The third pane shows you every file that was changed in the chosen revision. Double-click on your file in this pane and you can see the changes made in that file.
As remarked previously, Tortoise has a "Blame" functionality which is often very insightful but it can only show the most recent edit on each line of source. And if someone has deleted lines, you don't see their change at all.
Edit:
If you're willing to go to the command-line tool and bash, this question has been answered before: How can I view all historical changes to a file in SVN
On a Windows PC, the best way to get bash and command-line SVN is by installing Cygwin.